How to throw a cross
The rear-hand straight — the first power punch you learn, driven from the floor through the hip.
Action
From the guard, push off the ball of the rear foot and let that foot pivot inward. The rear hip turns through, the rear shoulder comes around, and the rear fist travels on a straight line across your body, just past your own chin — that crossing line is what gives the punch its name. The lead hand stays high at the cheek the whole way, because your shoulders square up for a moment as the punch lands. Snap the hand straight back to the guard and re-set your stance immediately.
When to use
Almost always behind the jab: the jab steals a beat of attention, and the cross arrives while the opponent is still looking at the lead hand. It is also the standard counter when the guard drops, and the finish of most basic combinations — 1-2, 1-2-3, 1-1-2.
Advantages
The hardest straight punch you own. The force comes from the floor through the rear leg, hip and shoulder instead of the arm, so it lands far heavier than the jab while still travelling on a straight, economical line.
Limits
The big hip rotation squares your stance and briefly takes you off your base, so a missed cross is expensive — the counter hook is already loaded. Thrown cold from too far away it is easy to see coming, and a rear hand that stays low after the punch is the single most common knockout in boxing.