When you personally have made a repair to a machine and feel that it works better just because you worked on it. Even if the part of the machine you worked on would not make it work any better.
Jim: "I replaced the heater core in my car, now it seems faster..."
Sam: "I've heard of that before."
Jim: "Seems to run smoother too!?"
Sam: "That's Mental Horsepower!"
Ricers who do modifications to everything except their engine to gain horsepower and think there car is fast.
A ricer who has installed a fart can for exhaust, thinks he can our run a stock mustang 5.0 or a porcshe 911. They have mental horsepower.