It's easy, if you go slow enough it will warn you that you're going reverse, but only in grey text (doesn't change anything). When I see someone coming on the map, I start to accelerate, and I can build enough speed before they get to me to send them flying. If you drive slowly in reverse and the text turns red, brake and start moving again.
Another good way to rage is to send people in the sand, because with the fastest car they won't be able to get out and they'll have to pit, making them lose their position.
Btw, some servers allow you to go in reverse.