Slip editing is when you grab the leading edge of any clip, audio or midi and pull it back and forth.
By pulling the leading edge to zero it stops the clip from moving in that direction so one less thing to go wrong.
That's why I said to zoom in a bit so as to make sure the track doen't move ahead or back.
I always slip edit audio tracks to the very moment sound starts to eliminate unwanted noise.
Slip editing is cool because your not deleting anything. Your just hidding it or exposing it.