Gaster is the man behind the tree and he goes behind trees because Gaster is hiding the fact that he has a crush on Kris. When you go into an egg room, only Kris enters. This is so that Gaster can get alone time with them, even if it may just be a few seconds. Gaster hide behind the tree to hide his blush.

I tried to use it a little bit ago, and it crashed after 3rd attack. Also, sometimes I can't do certain actions because the site counts one button press as many sometimes. Perhaps it depends on the Switch version you use?