One literally causes his own defeat before a single weapon has been employed against him. Applying the principle of change in combat hinges on flexibility or the ability to change. Mentally one prepares himself before combat ensues to accept any eventuality that occurs. Combat is intense, brutal, and deadly. Physically one prepares for change by developing physical power and reactions.
Physical power is developed through forms, drills, and practice.
Whereas, responding and adapting in a fighting situation is built through the consistent practice and study of Chi Sau. Chi Sau will develop one's ability to receive the opponents attack, respond correctly, and attack with intense, forward energy. Chi Sau is the essential link between forms and free-fighting. Finally, one must remember that change is merely one principle in the totality of Wing Chun and not an end in itself.