to fill out the something somethings for the unaware audience -

if you get something into your bum fully, it’s very hard to get out on your own without hurting yourself, and your ass can suck that thing in in a second - so unless you’re an avid fan of going to a doctor to get something out of your butt, the object you’re penetrating yourself with has to have a flared base - that is a base at the bottom of the object much wider than what your ass can suck in, which will stop the thing from sliding in