When a broken wave holds you under until it has passed. If the next wave in the set reaches you before you surface, the scene calls it a two- or three-wave hold-down.
The hold-down is one of the reasons big wave surfing is never a solo sport. What makes it dangerous is rarely the single wave but the chain: if the next breaker in the set arrives before you've drawn a breath, one hold turns into two. In 2012 professional Greg Long was caught in a three-wave hold-down at Cortes Bank and was pulled from the water unconscious; his own conclusion was that his breath training kept him from panicking, yet he still went past his limit. Two numbers frame the situation: between two waves in a set there are often only 7 to 18 seconds for a rescue, and in water below 15°C cold shock shrinks a normally 60 to 90 second breath-hold to a few seconds. What often gets missed: breath training never replaces a safety team — and it is never practised alone.