The handstand is the basic skill in gymnastics. It is also the most important. My former coach Gary would tell me: "Gymnastics is just a moving handstand. Master the handstand and skills come naturally." Coaches often tell gymnasts to "Stay tight!" or "Tuck in your tummy!" From observing the physics of the handstand, one is able to see that these tips are actually enabling the gymnast to attain the position that is physically preferable.
The handstand takes a lot of strength to hold the body in the proper position. Although the handstand requires the whole body to be "tight" while in the position, the main areas of the body used are the shoulders, stomach, and hips. The handstand, like all other tumbling skills is performed in a straight line. If the line is broken then the skill is hard to control and lost, in this case the handstand has a lack of "holding" time. On earth we have a force called gravity. It causes us to accelerate towards the earth's surface at a -9.8 m/s. While doing a handstand gravity works against you. When performing the handstand your center of gravity must be directly over your hands, otherwise you will fall. Your back must not be arched or gravity will pull you over, the same goes with the hips being piked, gravity will pull you right back down. Newton's First law of inertia explains this. " A net or unbalanced force is needed to change the state of motion of an object." Once you have reached the peak of a handstand you would stay there forever if there were no gravity. What keeps us from falling through the ground? Newton's third law is the answer! "For every force on an object there will always be an equal and opposite force exerted by that object." Which means, whatever the amount of force you are exerting on the earth's surface, it is exerting the EXACT same amount back at you. We've established that doing a handstand is nothing more than a balancing act.
Furthermore, the handstand involves the concept of center of gravity. Handstands are difficult because it is hard to maintain the center of gravity in the line perpendicular from the floor. It is very easy to tip over on either side.