I've always used the wavy lines to adjust my RO skimmer gate valve. I got it second hand and never did see an owners manual. Mine tends to "bounce around" after I have cleaned it. I adjust the gate valve and then readjust it after an hour, and check it again (today, Just cleaned mine yesterday, Thanks for the remind.)
These can also bounce around when adding messy food, medicines, or just putting a hand in the tank, ime.
Edit, after the video loaded, yours seems to be bouncing a lot more than mine does, like it is hitting a wall of some sort. I don't know why it would do that? Is the skimmer intake pump sucking air as the level in the sump gets lower? If yes, you need more water depth in the sump to avoid this.