The difference between an overlay and an insert edit is when you use overlay, the 1st clip will keep playing during the 2nd clip. When you insert the clip, the 1st clip will stop where it's at, and after the 2nd clip has ended, the 1st will pick up from where it left off. You would use an overlay when you're making interview-like videos, when it would look funny if you cut the clip off,while you’d use an insert edit when you need to stop the fist clip, like in a flashback (just some examples). In this case, the insert edit works better because you can follow the horse's path and know where it's going to be.