The windmill, or bent press, is an excellent stretch and strength move, working the entire body with a particular emphasis on trunk muscles, hamstrings, and shoulders. It requires, and helps develop, flexibility, strength, stability, and coordination.
First go through this movement without a weight in your hand. Once you get the foot position, hip placement, and control of your hand overhead nailed, slowly work up to a weight that your strength and flexibility allow.
Take a weighted object, such as barbell, dumbbell, or kettlebell (or sandbag, rock, small child, etc.), overhead in one hand, with your arm and shoulder fully extended. Keep the weight light at first, and make sure you master controlling the load and can keep your arm locked out at all times before increasing the load. Set your feet so they are parallel, toes pointing in the same direction. Play with the distance between your heels—everyone is different, though too wide is not good. Exaggerate pushing your hip out to the side (on the same side as the overhead arm). Place your free hand on your leading leg. Eyes constantly fixed on the overhead object (think of supporting a cup of hot tar in your hand), let the hand on your leg help control the descent into the stretch (you may also limit the depth and speed of the stretch with the hand on your leg).
Eventually work down to the laces on your shoes. Progress gradually until you can lay your palm flat on the floor.
For more information, see Arthur Saxon’s chapter on the bent press