While having sex with a girl from behind (doggystyle, with both hands on her ass cheeks) you use your right thumb to apply light pressure to her asshole (the "B" button) therefore initiating turbo mode. The pace of copulation will noticeably increase along with a pleasant turbo moaning sound.
Be careful though, too much and she will overheat with uncomfort and get bitchy. You will then be sidelined for a while while she tells you to "stop fingering my asshole". Fucking may resume, however you may have already lost the race.
Use sparingly for that extra boost of speed, preferably while coming inside of her.
We could tell when Rod was Excitebiking his girlfriend because the banging got louder, the pitch of moans changed, and eventually we would hear him giggling as she yelled at him.