Unless the implement used is very narrow, most smacks aimed at the bottom hole will tend to impact on the innermost part of each buttock, just above the hole itself. Which might be what you’ve seen in videos.

If you are spanking the bottom hole, the same advice applies as given for pussy spankings – namely smack slowly, don’t use too much force, and the recipient is always in control. Done right, it can be extremely erotic, just don’t spoil the moment by escalating too quickly.