To give maximum effort or energy to an event.
To make love with added energy.
To make an engine give its maximum performance.
The train conductor decided to pour the coal to the engine when he went up the hill.
When I saw Sandy last night, I didn't just (make love). I spent 3 hours pouring the coal to her. I had not seen her in weeks!
I made it here in less than an hour. I was pouring the coal to that car.
The engineer wanted to pull more cars, so he told the conductor to start pouring the coal.