The pressure applied across the piston during the combustion which transfers force to the crank to be multiplied by stroke during one power stroke is Torque, HP is the sum of all those power strokes over time rpm. If your engine makes 400 lbs-ft at 3500 rpm it's making 400 lbs-ft per ever two revolution so 50% of 3500 = 1750 x 400 which is baked in the torque x rpm / 5252 = hp. If my engine makes 200 lbs-ft every two revolution, I'll have to spin 4 revolution for every two of yours in the same amount of time so twice the rpm to make same hp cause I'm basically making 400 lbs-ft in 4 revolution instead of two.