Let's try with pictures and hope this helps.

Above is the start of an animation channel, defualt start at frame 1 and at frame 4 we have a downward movement and frame the end of the downward movements.

At frame 10 I added an extra keyframe to stop the curve from heading downward on frame 8 as I added in frame 21. SO far there are 5 keyframes four I addded and the defualt one at frame 1.

Notice when I add in key frame 22 the line drops lower between frame 12 and 20.

I can stop this by putting in keyframes at 19 and 20, see above. You do this by clicking on the red graph line on frames 20 and 19 before putting in key frame 22. What that does is puts in the value at the moment the time line is at, and as long as you do not chage the value the curve will remain as it is. However notice what happens when I place another key frame at frame 43. The graph jumps up to peak at frame 28 before heading to where we want it. This is what causes jumping and what putting in the extra key frames help suppress. You can minimize it by placing a key frame at 23, however that starts to add up a lot of keyframes. Look at image 5 below.

Here I removed the extra key frames 19 and 20. and created another layer that starts to frame 21 and ends at frame 40. It is set to replace mode.

At this point I selected the channel, body part, or body that I want to copy and used ctrl C on the base layer. Select the new layer and select the same channel,body part or body that you did the ctrl C on and do a ctrl V on the same frame. Next two pioctures.

ctrl C

ctrl V

Add in the one extra key frame at end of the layer and now the curve will follow that curve during the layers run duration

will add the extra picture in the next post

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13