A warped wood cutting board can almost always be flattened again, because warping is just moisture sitting unevenly inside the wood rather than damage to it. A cracked one is a different problem. Once the fibers have physically torn apart along the grain, soaking and clamping won't rejoin them, and the board is done.
Why a flat board turns into a bowl
Wood is hygroscopic — it pulls moisture from the air and gives it back as humidity changes, and it swells and shrinks far more across the grain than along it. A board only stays flat while both faces hold roughly the same amount of water.
Wash a board and set it down wet-side-up on a rack, or let it sit flat with one face against a damp counter, and the two faces stop matching. The face that took on more moisture swells wider than the drier one. Since both faces are still one solid piece of wood, the swollen face has to go somewhere, and it becomes the outside of a curve — the board cups with the wetter face bulging out and the drier face pulled concave. That's why boards so often warp right after a hand wash: one side got soaked, the other barely got wet at all.
How to flatten it back out
Since the problem is a moisture mismatch, the fix is to even it back out. Wet the concave side — the hollow face — with a damp towel or a light spray, then lay the board flat with something heavy on top and let it dry slowly over a day or two, flipping occasionally. As the dry face catches up in moisture and the whole board dries at the same rate on both sides, it settles flatter than it started.
This works because the wood fibers themselves are still intact; only their moisture content changed. It does not work indefinitely. A board that gets soaked unevenly over and over develops a set the wood remembers, and each round of flattening buys less improvement than the last.
Why a crack is a different kind of failure
A crack starts the same way a warp does — one part of the board losing or gaining moisture faster than the rest — but taken further. Dry a board too fast, near a heater, a dishwasher, or direct sun, and the outer layer shrinks before the wetter wood underneath has a chance to follow. The surface is being pulled tight by wood that hasn't caught up yet, and if that tension outruns what the fibers can take, they separate along the grain instead of stretching.
That separation is mechanical, not chemical, and there's no re-wetting step that undoes it — torn fibers don't grow back together. A hairline check on the surface is mostly cosmetic. A crack that runs any real depth is worth retiring the board over, because it opens a seam that a knife can drive food and moisture into and a scrub brush can't reach.