The underwater phase after the start and every turn: one deep pull to the thighs, one dolphin kick, one breaststroke kick. The fastest stretch of any breaststroke length.
Below the surface wave drag disappears, which is why the pullout is faster than anything you can do on top of the water. A 100 m race in a 25 m pool has one start and three turns — four pullouts. Two tenths on each is nearly a full second, without a single extra stroke. The most common fault is surfacing too early and giving that speed away; the second is a dolphin kick that happens somewhere at random instead of towards the end of the arm pull, as the hands shoot back. Find your breakout by speed, not by feel: surface when your underwater pace has dropped to your racing pace. Two things that get missed: under the rules your head has to break the surface before the hands reach the widest point of the second stroke — check your federation's current rulebook. And a pullout means holding your breath: never practise it to your limit, and never after hyperventilating.