When you check "relax constraints and dimensions", the constraint solver is disabled, as if you'd deleted all the constraints (except point-coincident constraints). The constraints aren't actually deleted, though; they're still drawn on the sketch, and when you un-check that box, the constraint solver will attempt to satisfy them again.

This feature is sometimes useful when a sketch has finitely many solutions, and the solver found the wrong one. (For example, you might draw a cyclic pentagon with equal side lengths, expecting a regular pentagon, but instead get a five-pointed star.) The solver picks whatever solution is closest to the initial position of the geometry; so to fix that, you can disable the solver, drag the geometry closer to your desired solution, and re-enable the solver.

May I propose to just copy your explanation into the reference manual because its really clear.