As far as I can work out, jostling your rival is, for example, when being right behind your rival and pressing 'Circle' to trip him up and make him lose rings.
You can't be him in that game.
It a second version of sonic rival exist there be more charcter better bosses and better stage and more story mode.
When selecting character press Triangle
You have to unloock diferrent Cups and play Multiplayer to unlock character costumes
Tap square button continuaslyJust colect rings or defeat enemies and watch the signsture meter when it's full press the square