When fine sediment lifts and visibility drops to zero within seconds. Usually it is not the cave that causes it but your own fin kick.
A silt-out is rarely an act of nature — usually it is your own fin kick. The flutter kick you learned in open water pushes water downwards on every stroke, and over fine sediment that is enough for zero visibility within seconds. In accident analyses the silt-out sits at the start of a chain: sight gone, orientation gone, gas gone, panic. Which is exactly why it is not an emergency in itself — with a hand on the line and a calm head you can feel your way out in zero visibility. The real problem is a silt-out plus no line contact. Prevention means relearning: frog kick instead of flutter, the smaller modified frog in tight passages, and horizontal trim as the precondition for both. Two things get missed here. You take away your team's visibility as well, and the cloud does not clear on command. And clear water tends to make it worse, because it makes you careless.