They can add health or damage either to the part they are assigned to or to a different part. For example, you can have a Classic that gives 10% Attack to Chainsaw, or the same Classic that gives 20% Health to Bigfoot wheels.
You can increase the Magic Bonus of the part by placing this part on a Bet and winning. Longer Bets give bigger bonuses.
Note: You can only increase the Bonus up to 60%.
Parts with Magic Bonuses are called "Super Parts." The parts with a 100% Bonus are called Legendary Parts. They are very rare and can be found in certain Supply Boxes.