what are inverse kinetics good for?

So far, the only use I can find for them is making me have to remember to turn them off everytime I start a new animation.

IK lets you move the hand or foot and lets the arm or leg adjust automaticly. A good example would be if you were doing a tai chi animation and needed the hand to move in a smoth line. With IK you just move the hand. Without IK you have to adjust the arm and wrist and shoulder to move it, and then it won't be going in a line so you'd have to change it in the middle. It does cause problems sometimes making joints bend weirdly, but you can turn it on or off at any time. I leave it off for the hands most of the time and only turn it on for specific movements and then turn it off after I've used it. The feet I leave on if the avatar is standing so I can move the hip and the legs will adjust on their own. I turn it off for a leg if it is moving or the knee gets weird.

For accuracy's sake, it's "kinematics", not "kinetics".

Tuff's explanation is right, but just to expand a little, kinematics in 3D animation come in two types, forward and inverse. With forward kinematics (FK), body parts are moved by adjusting the individual rotation values for each specific joint, one at a time. For example, to have your figure raise his knee using FK, you'd first rotate the hip/thigh joint, which would raise the entire leg. Then you'd rotate the knee/shin joint to bend the knee.

Using IK, the process is much simpler. Inverting the kinematics loosely translates to beginning with the end in mind, rather than going step by step. To raise the figure's knee, you'd simply grab hold of the foot, and lift it up. The joints in the leg will automatically bend to compensate.

There are serious advantages and disadvantages to both methods, which is why Poser and other animation programs allow you to toggle between the two. For some silly reason I can't fathom, the makers of Poser saw fit to remove the term "forward kinematics" from their vocabulary, instead choosing to call it turning off IK, which really doesn't make much sense. It should be an FK/IK toggle, not an on off switch for IK.

Anyway, the biggest disadvantages to IK are twofold. First, you're completely at the mercy of whomever rigged the figure. If they did a good job setting up their IK handles, rotational restrictions, pole vectors, etc., (which professional Poser content creators usually do with reasonable competence) then it can work very well. If the figure is poorly rigged, then you can pretty much count on IK being totally useless.

Second, IK can be clumsy. It's demands a major sacrifice from your level of control. For example, to go back to the raising the knee thing for a second, what if your goal in the animation were to simulate breaking the knee by bending it backwards? If your IK rig is (properly) set up for normal human motion restrictions, you wouldn't be able to do it.

A more common example that happens in Poser all the time is body parts can refuse to move. With IK turned on for the legs, when you try to move the body as a whole, the feet remain planted to the ground. The reason is that under the IK scheme, the feet control the legs, which is a backwards (inverse) way to go about it. In real life, if someone pushes your pelvis across the room, your feet will follow. Real life locomotion is all FK; to lift your knee, your upper leg does the work, not your foot. With IK turned on in Poser, and in most animation programs, the feet will try to stay put until you specifically tell them to move, which of course is absolutely unnatural.

The advantage of IK is it's a time saver. You may not always want to think about every joint rotation every time. There are plenty of times when you'll want to just lift that foot, and let the leg do what it will.

For example, let's say you want your figure to kick a soccer ball. It can be quite a brain teaser to calculate exactly the right rotations of the hip, knee, and ankle in order for the foot and ball to make contact. With IK, all you have to do is drag the foot to the ball, and the rest of the leg will follow along.

In creating animations, you'll find that switching between IK and FK constantly is what you'll need to do. Neither one is best for all things.

I hope this info has been helpful. Happy animating.

I imagine they already covered this (not read the other posts) so here's just a quick explanation of the terms: forward kinematics is when you have to move each body part individually to achieve a motion, inverse kinematics is when you just move the end of a chain and all the other limbs move appropriately. The classic example is the leg; with FK you have to bend the upper leg then lower leg, but with IK you just move the foot around. The names derive from the heirarchy linkages of foot linked to lower leg linked to upper leg linked to hip. Forward is going down the chain, inverse is moving up from the end of the chain.

Actually, if you think about it if someone pushes your pelvis back, unless they have pushed your center of gravity beyond your feet your feet aren't going to move at all. They will stay absolutely in the same spot. Even if they do push your pelvis back far enough that you are forced to move your feet they will stay in the same spot until the 'lift'.

While it's true that most motion is, at its core, forward kinematic any time external forces (such as friction) are governing the motions of a body part then you are looking at a situation where the end effector (such as the hand) is driving the position of the joints.

Of course that assumes a total override of the musculature like a hand being used to position the arm of a sleeper. You can have plenty of situations like climbing a rope where the position of the hand is a balance between the external forces and the internal forces.

All of that said, even though a limb may be moving under pure forward kinematics it is often beneficial to use an ik setup. This is because the brain usually doesn't view the body in FK terms. An excellent example is a poking motion made with the finger. The brain wants the hand to move forward in a straight line. It isn't really concerned with the placement of the shoulder, forearm, or bicep. While those parts of the limb are actually driving the motion the limb itself is still behaving very much in an IK fashion.

I/K (inverse kinematics) is most useful placing feet, and placing hips relative to the feet, which is nearly always how it's done outside of SL. You can animate a walk using nothing but forward kinematics, but most animators consider this an unusually cruel form of torture. Even the simple act of an avatar bending its knees becomes a registration nightmare - the feet invariably slip.

Everything above the waist, however, is normally done with F/K (forward kinematics) because all the motion is relative to the waist instead of the feet - unless you're animating a character picking up an object in one hand, which in SL we're unable to do.

Normally one builds the animation up starting with the feet and the hips in relative movement to the feet. If you don't get this part right, you'll be counter-animating the upper torso to correct your mistakes, and you'll never get a clean action.

Edit: I'll add to this that trying to animate in Poser compared to other tools like Maya, Lightwave, Max, Filmbox or practically anything else is like trying to paint with a brick dipped in dye. It's the worse possible tool for this task, because of the lack of features critical to creating usable rigs.