Frame modification problem in poser 6

Hello all.

I am still a newbie animator but I think I got round static poses quite well. My problem now is animations. Let me explain :

on my first try for animation I noticed something and since then I am pulling my hair off trying to figure how to get rid of that. When I mod the figure on a specific frame certain modification also apply not only to next frames, but also to previous frames messing all my work and giving me the double of work with hazardous results .... anybody can tell me how to disable this function ?.

Thanks in advance.