If you want to have frame 40 be at 100% and then frame 41 be at 0 put a keyframe at frame 40 and frame 41.
If you want to put a keyframe at frame 10 and another at frame 41 and you want to values to change on frame 41 instantly to the new value then use Toggle hold keyframes by context clicking the keyframe at frame 10 an make the selection..
This is BASIC keyframing technique. Have you goine through any of the getting started training materials or searched the help files for keyframing???
Thanks, that's what I ended up doing, I'm glad it is the approved technique. And yes, I have been through tons of stuff; but it seems what the kids want is easing so most of the keyframing and interpolation pages are about that. Sorry to put you out.