To get this you must be playing in a team race (eg. Sonic, Tails and Knuckles all in one team whilst you control one of the characters). When a teammate is driving slowly or has spun out, skim close past them to give them a "Skimboost"
Upon performing right you will see some text on the right of your vehicle with a golden star saying "Skimboost" alerting you to the fact that it worked and registered.
I created this video to help:
Hope this helps.