Volume constancy
Plastic deformation keeps volume almost constant. If the width stays about the same, thickness times speed must match on both sides of the rolls: h1 x v1 = h2 x v2. The exit speed readout is simply v1 x h1 / h2.
Draft is the thickness removed in one pass, h1 minus h2. Reduction is draft as a percentage of the entry thickness.
Try it: set the entry to 200 mm at 1 m/s and push the reduction to 50 percent. The strip leaves at 2 m/s, but the 100 mm draft is far beyond what the rolls can grip, so the bite readout says no.
How much the rolls can bite
A standard result says the largest draft the rolls can pull in is about the friction coefficient squared times the roll radius. This model assumes a friction coefficient of 0.3 and a 500 mm roll radius, giving about 45 mm, a simplified teaching value. Real mills rough a slab down over many passes, then finish it on a tandem line of stands running ever faster.