Computer Science is a type of Math. With some mathematical proofs, you can explain the "trick" in a sentence or two, and with others you can't. For example, to prove the formula for the area of a right triangle, the trick is form a rectangle with two copies of it. But you can't explain the trick for proving the Pythagorean Theorem briefly. That's also the sitch with heapsort. Proving it involves several non-obvious steps, and you just have to slog through them.