Two things at once
Heating forms scale on the slab. Scale and steel shrink at different rates: a high-pressure water jet cools the surface abruptly, the interface is torn open — thermal shock.
Tearing alone is not enough: the jet must also carry enough impact force to blow the loosened scale away. Shock separates; impact removes.
Primary and secondary scale
Primary scale (thick, dense) is removed before roughing; secondary scale (thin but tightly adherent) before finishing. The latter needs higher impact and more uniform nozzle layout.
Why the temperature window matters
Too hot: scale is soft and sticks. Too cold: thermal shock weakens. Descaling points sit where both work — which is why retrofits must consider the whole line, not just the nozzles.
General industry knowledge; confirm parameters against your process and measurements.