I'll preface by saying I know very little about the inner workings of automobiles. I can own and maintain one. That's about it. For this scenario the car in question has automatic transmission. I have to drive to the mountains this weekend and I've been mentally going over my route. I am aware of engine braking. I know that I should manually switch to gear 1 on my automatic for steep slopes in order to retard acceleration. That way I do not ride my brakes. I know that after a point this stops fuel flow to the engine and a vacuum is formed or something. What I can't wrap my head around is why does RPM increase? RPM increase under normal circumstances means increased speed. But in a low gear during engine braking you are retarding speed. Why does the RPM rise? Is it because the wheels are turning which is transferring energy to the engine via the transmission? I don't know. It's not important. I'm just curious and trying to picture this all taking place internally. Any answers are welcomed.