Most people will react immediately to a ball coming their way, by moving their arm and hand in order to catch it. Choosing not to allow movement of your hand or arm, inhibits part or all of your usual reaction to what can be a strong stimulus. Some people find this easier than others. It is often a surprising experience to discover that your body will respond to your intention to move without the application of any conscious effort. The mechanisms that lie behind this are beginning to be demonstrated in scientific research, but for now our understanding is that the supporting mechanisms of your posture and movement are essentially organised through automatic reflex actions in your musculature.
Every movement has a element of reflex action, involuntarily acting to support the movement.
It is a complex system, but the experience of movement supported effectively with integrated reflex actions is one of extraordinary ease and simplicity. When you see someone who is very skilled in a particular activity, they make it look easy to achieve. In fact, if you ask them, you'll find that their experience is that it is easy - even though it may have taken years of training and work to reach that skill level. Example You may be familiar with a child's game, where you stand in a doorway, and press your arms out against the frame for a minute or so. These directions are to be projected or intended, but it is of primary importance that no attempt is made to do them directly.