Evading with a lateral head movement so the incoming punch passes just by — without opening your guard.
Slipping is the most economical defence because you stay in punching range and can counter immediately, unlike stepping back. The movement comes from a slight rotation and flexion of the torso, not the neck: rotate the shoulder with it and let your head travel a couple of hand-widths off the line — that's all it takes. Two mistakes dominate: moving too far, which costs balance and counter range, and closing your eyes or looking away while doing it. Eyes stay on your opponent. Someone who slips cleanly saves a surprising amount of energy over a round compared with someone who blocks everything.