Comment by kibwen

Someone else here compared it to regenerative braking in cars, which is what made it click for me. If you spend energy to accelerate, then recapture that energy while decelerating, then you can manage to transport yourself while your net energy expenditure is zero (other than all that pesky friction). On the other hand, if you spend energy to accelerate, then shed all that energy via heat from your brake pads, then you need to expend new energy to accelerate next time.