Interference

Interference is what happens when two routes toward the same reading contribute amplitudes that meet. Matching signs reinforce that reading, and opposite signs can cancel it, which is why a second Hadamard puts the |+⟩ state back to the state certain to read 0.

The slogan about trying every answer at once has nowhere to write a minus sign. Malcolm considers that a full review of the slogan, and he has stopped delivering the review at a shout.