Because if Alice fails to get H->T, then she has to be back at the first step, H, ready to flip again for H->T If Bob starts on H and gets T, he needs to conti

Because if Alice fails to get H->T, then she has to be back at the first step, H, ready to flip again for H->T If Bob starts on H and gets T, he needs to continue flipping until he gets back to H.