Here's a simplified side view of a pelvis (and legs and some ribcage and spine).
Here's what happens when you sit on your tailbone on a chair, or couch, or the floor. When you put pressure on your tailbone, it pushes it into your pelvic cavity and puts slack in your pelvic floor and makes it like a hammock.
Skeletal muscle doesn't like to be floppy or saggy, however, so it will actually rearrange itself by closing the gaps to be taut again. This isn't the same as contracting a muscle to make it shorter though. This is the muscle changing its resting length. If the resting length of your bicep changed like this, you wouldn't be able to straighten your arm. It would just be bent all the time without you flexing to get it there. The muscle becomes permanently short (unless you started stretching it back out again.)
Here's what happens during a Kegel. The pelvic floor contracts into its shortest position and tugs the tailbone even further into the pelvic cavity over time. Often, people that do Kegels are doing them because they already have some kind of pelvic floor problem, which is probably a too short or tight pelvic floor. So when you do a Kegel, you contract the muscle even further, to it's absolute shortest length. If you do this enough, and you never lengthen the muscle back out, the muscle will eventually arrange itself so that the gaps stay closed. Once the gaps are closed, there is no possibility to generate force! This is where really bad pelvic floor disorder comes from, including things like urinary incontinence, pelvic organ prolapse, chronic pelvic and low back pain, stuff like that.
Even if you're not doing Kegels, if you go around with your pelvic floor really tense all the time, it creates the same problem. Right now as you're reading this, see if you can relax your pelvic floor without peeing. If you were able to relax and not pee, you were tensing your pelvic floor too much, and shortening it. It's like doing an all day low intensity Kegel, which is not good for the health of your pelvic floor. Your pelvic floor really only has to be tense enough to hold in your pee, you don't need to clench it all the time.
So, what should we do instead of a Kegel for a healthy pelvic floor? There is actually a lot you can do to fix this problem! My top 4 tips (out of like, a gazillion) are:
1. Stop sitting on your sacrum. (click) Try to stand up as much as possible.
2. Stop doing Kegels, and check in with your body as often as you can to make sure you're not over-contracting your pelvic floor.
3. Stretch your hamstrings. A LOT. Here's how you can do it every time you bend over. (click)
4. Use your glutes! The glutes offer counter tension on the tailbone, which pulls the pelvic floor taught again without any muscle rearranging and shortening. (here's a sweet class you can buy and do at home. click)
For further information, check out this blog post (click) over at Katy Says. There is so much good info over here!