A skydiver's body is in a delicate balance. Gravity tugs her downward with a constant, relentless force, while air resistance pushes her upward. The faster she falls, the more air resistance there is, so eventually these two forces cancel out. Then she falls at a constant rate, her terminal velocity.
To link up with other skydivers in mid-air formations, skydivers need to control their terminal velocity very carefully. After all, if you're falling at a 110 miles per hour, and your friend is falling at a 120, there's no way you can link at all.
As it turns out, your terminal velocity is determined by exactly two things, your weight, and the amount of surface area you expose to the ground. While a skydiver can't do anything about her weight mid-fall, she can control how much area she presents downward. Stomach down, with arms and legs spread out in a kind of belly-flop, the average skydiver is likely to fall at around a 110 miles per hour.
By bending arms and legs or angling her body, presenting more or less surface area to the ground, she can change her rate of fall, slowing or speeding up her terminal velocity around 10 or 20 miles per hour. To fall as fast as 200 miles per hour, a skydiver goes into either a diving or a standing up posture. This presents the least surface area downward, so her terminal velocity is highest.
When a skydiver opens her chute, she's increasing the amount of surface area she presents to the ground dramatically. Her terminal velocity slows enough for a soft landing.