But since most people don't have a clue as to the inner workings of this pelvic floor, let's get to the specifics already!
This drawing is one view of the pelvic floor. To get this view, you would stand on the floor looking up between a person's legs that were spread WIDE apart. (You also need x-ray vision.) True, there are obviously differences between the way this looks and functions between men and women, but this gives a good idea of the orientation and location.
When you picture these muscles as living, breathing, 3-dimensional moving parts, imagine them in the shape of a bowl. (From the same point of view, you'd be looking at the underside of the bowl.) They are rounded, and form a hammock, or a sling that connects to your sitz bones, tail bone, and pubic bones. When those bones move, as they do in walking, climbing, dancing, sexing, running, etc., so do the muscles that form this bowl.
Within this bowl is held a lot of important goods: nerve endings, vital organs and glands, and sometimes babies. Basically, think of everything underneath your breathing diaphragm (under the lungs) as being upheld by the pelvic floor. It is the last stop before everything hits the ground. I'd say that makes it pretty darn important.
1. Personally, I don't use a tennis ball, EVER. It's too small and too hard. No good. Instead I sit over a small (and soft) franklin ball or a foam roller. (Orient the roller to run like a train between your legs and fold your legs under you on either side of it.) If this feels squeamish and uncomfortable to you, that's normal. Accommodate yourself by putting your hands on your thighs or the surface you are sitting on and pushing into them, thus taking some weight off the ball/roller.
3. Now keep doing it, and notice if you are in fact gripping a whole lot around your anus... Ok, now see if you can engage the front of your pelvic floor without yanking on your a-hole. Pretty difficult for most people, but with practice and awareness this will start to change.
Forgive me while I get specific here. But there's a couple of big advantages to NOT being anal retentive. Firstly, check out this picture. You can see the anal sphincter highlighted in green. And you can see how it is interrelated with the muscles of the vagina, and connects to the tail bone.
So, from a muscular perspective, over-gripping the ass will under-train your ability to fire the rest of your floor, specifically the vaginal muscles. The motto here is "don't let your butt-hole dominate the conversation (of your pelvic floor)."
From a bony perspective, this same pattern will pull the tail bone down, and limit the way your spine and sacrum glide in relationship to your pelvic bones and femurs. This motto might be "let go, and be free!"