Resistance training using the body itself as the load, progressed by changing leverage, range and tempo rather than by adding weight.
It works for the same reason weights work: the muscle responds to tension and proximity to failure, not to whether the resistance came from a barbell. The problem is progression, since the load is fixed.
Four levers replace the weight plate. Leverage: incline push-up to flat to feet-elevated to ring. Range: half squat to full to deficit. Unilateral: two legs to split to single. Time under tension: a three-second lowering phase makes a push-up materially harder without changing anything else.
A concrete progression: 3 x 12 incline push-ups on a bench, then 3 x 12 on the floor, then 3 x 8 feet-elevated, then 3 x 8 with a three-second lowering. That is four to six months of progress for a beginner with no equipment.