Your Hollow Body isn't Hollow
One of the biggest issues I see in gymnastics training is a lack of hollow body strength. Hollow body is probably the most foundational shape used in gymnastics, so it's essential to master it, strengthen it, and continuously practice it. Contrary to popular belief, the most important part of staying hollow is not a crunch of the abs. It's a posterior tilt.
Gymnasts describe this as tucking the tail or pulling the hips underneath you. Try this: Put your hand on your belly button and arch your back as much as possible. Then pull your hips the opposite way- you should feel your abs tighten. What you just did is a posterior tilt. I've also heard the pelvis described as a wine glass, and a posterior tilt as spilling the wine behind you.
How do we make sure this is happening in a hollow body? Easy. If you have a posterior tilt, your lower back will be pushed into the ground. If your back isn't touching the floor, you're probably arched.
Let's do it: Start near a wall and lie on your back with your knees bent and your feet flat on the floor. Put your hands on your hips and tilt your pelvis forward until your back is on the floor. Crunch your abs, pulling your shoulder blades off the floor. Then extend your legs to the wall about 6in. off the floor and PUSH the ball of the foot into the wall, hard. Your lower back should still be on the floor, and you should look like a banana. This is a great drill for achieving hollow body. Once you've strengthened the position, start to move away from the wall, still mimicking the "push" with the feet into an invisible wall.
For beginning students new to movement/obese/special needs/stroke/back injury/etc, OR as use for a warmup: A more beginners-friendly version of this is to simply hug the knees to the chest with the arms, bring the forward to the knees, and try to hold there. Then students can slowly start to take the arms off the knees, and even more slowly unbend the legs. The trick is that compressing the knees to the chest will always elicit a posterior tilt, as long as the knees are legitimately held at the chest. This can be applied to any movement that requires a hollow body- pullups, toes to bar, handstands, etc. If maintaining hollow is difficult, bring the knees to the chest.
Once you're able to consistently find a hollow body in your skills, your body will move as one piece, instead of a bunch of disconnected arms, legs, and torsos, making all your skills more efficient, easier, and of course, prettier.