Trimming shape layer disappears all keyframes
Hello,
I do not understand something. From experience, I know where your keyframe ends you can cut /trim the layer there (where your animation ends). and it will appear as it is in composition. I am learning about 'repeater'. and create a starburst animation using the trim path. My composition is 10 seconds long. starburst animation keyframes (trim path start and ends keyframes) took only 2 seconds. So I thought I will cut the layer where the keyframe ends and copy-paste it to creat several star burst animations! But as I cut the layer all animation stops and the keyframe disappears! 
Why does this happen?
and another question is there any relation between pre-comp duration and main composition duration?
I mean suppose my composition is 10 seconds long. and if I create a precompose taking some layers what will be its duration? 10 seconds?
Thank you.
