Bear in mind torque has no movement. If you put 100 lbs of force on a 1 ft. wrench, you have 100 ft/lbs of torque even if the wrench doesn't move.
HP adds in the idea of torque and "speed". You can have torque without movement (Force) but no HP because of no movement. As soon as you start moving the wrench with your 100 ft/lbs of torque, you now develop HP. The faster you turn with that wrench with the 100 FT/lbs, the more HP you are developing. SO, peak HP is the point where the AVAILABLE torque from the engine (Which varies because of too many things to go into) and the speed the engine is turning is optimum.
The confusion is from the way torque and HP develop in a recip. gas engine. An electric motor has max torque at stall (Steam Engines Too). As RPM increases, Torque drops but HP goes up. Electric motor have a more linear HP/Torque curve that is a little easier to understand.
The reason your car slows down up a hill is the torque isn't great enough to maintain the force necessary to maintain the speed (hp). As the car/engine slows, HP drops, but torque increases (if your engine RPM is above torque peak). If the torque increases enough , you maintain the new, lower speed (less HP). If you are below the torque peak, you continue to slow down unless you downshift to get above the required torque level.
