What actually gets you disqualified in competitive swimming?
Breaking the technical definition of the stroke, and the rules are far more specific than spectators realise — which is why disqualifications frequently look invisible from the side of the pool.
The starts and finishes, which produce a large share of disqualifications:
A false start. One movement before the signal, with no second chances — modern rules allow no warning.
Early relay takeover, where the incoming swimmer's feet leave the block before the finishing swimmer touches. Measured electronically to hundredths, and a relay can be disqualified for a takeover nobody watching could see.
Incorrect finish touch for the stroke.
By stroke:
Butterfly — arms must recover over the water simultaneously, and the kick must be simultaneous. Two hands must touch the wall at the same time at turns and finish. A single alternating kick is a disqualification.
Breaststroke — the most technically policed. Simultaneous symmetrical movements; hands may not pass beyond the hipline except in the first stroke after start and turn; only one arm pull and one kick underwater after the start and each turn; a two-hand simultaneous touch; and — the frequent one — a scissor or flutter kick is illegal, which swimmers do unconsciously when tiring.
Backstroke — must stay on the back except during the turn, where a specified sequence applies; and a head-first touch on the back at the finish.
Freestyle — few stroke rules, since anything is permitted, but the turn and finish require a touch.
Universal to all strokes:
The 15-metre rule. A swimmer's head must break the surface within 15 metres of the start and each turn. This exists because underwater dolphin kicking proved faster than surface swimming, and unrestricted it would have changed the sport into an underwater one — a genuine rule change driven by technique outpacing the event's definition.
Not touching the wall at a turn, and walking or pushing off the bottom.
Judging is by stroke and turn judges positioned along the pool, with the referee confirming.