Burst Teleporting / double jumping

Anyone know how to do these scripts? an example would be tapping pgup twice for a double jump, or holding down pgdown, press a direction, and be able to do a short burst of speed in that direction. Ive used llSetforce and llApplyimpuse, but maybe I am getting it wrong, any ideas, tips, or scripts would be greatly appreciated.

Here is the superjump script from a script library. You can mod it to put in an on/off switch.

default
{
     attach(key avatar)
     {
          vector force = <0,0, llGetMass() * 6.2>;
          llSetForce(force, TRUE);
          if(avatar==NULL_KEY)
          {
               llSetForce(<0,0,0>,TRUE);
          }
     }
}