Negative motivation is essentially based on fear and anxiety. The idea behind it is that athletes and others will achieve more and more when their weaknesses are reinforced, when they are trained to be afraid of failure, and when an atmosphere of competition and mistrust becomes the norm. Negative reinforcement however, does not create optimal performers. It creates athletes/performers who are afraid to fail...which is the main ingredient for failure.
Positive reinforcement is based on focusing on the positives of performance and expanding on them. Human beings respond to praise and challenge. Optimal performers want to know how they can get better, but the focus needs to be on framing the conversation in positive terms. And...the athlete/human being must be directly involved in the conversation. In other words...optimal performance happens when the athlete sets their own achievement goals. The coaching/mentoring becomes about helping them get there.