[sheepdog 44]

Think of it this way. If you bleed speed going up your trading kinetic energy (speed) for potential energy (Height with gravity). Going down down hill, you want to trade your potential energy for kinetic energy and regain your original speed. That is the most efficient way.

Say your going 55mph and you want to get to the top of a hill. Driving with load allows you to use the least amount of energy to climb that hill. You crest the top at 40mph, and use your throttle to accelerate downhill. It's more efficient to accelerate WITH gravity.

Now if you keep a steady 55mph up that entire hill you use more fuel because energy consumption is a function of speed (and something else can't remember). Ignoring engine load, aerodynamics and rolling resistance, going 40 mph up a hill requires less energy than, going 55mph, than going 70mph. The faster you go up hill the more energy it requires.

Now your just fighting gravity which is pointless. Think back on how much more power you have to use to gain 5mph uphill. That same power (generalizing without physics equations here) will get you 10mph acceleration on the flat, and 15mph going downhill. The more you fight it the worse it gets.

If you use you power with gravity, you can convert that 15mph going downhill to potential energy going back up the next hill. This is why pumping on a swing swings you faster and higher. I like to imagine as rolling marbles.