A single oversized chip thrown in the pot = a call, unless there has been no bets. Then it is considered as a bet.
If he says raise before the chip is thown in the pot, or while the chip is still in the hand, then the chip is considered a raise. unless the chip is under 50% of the minimum raise, then the player has to raise the minimum.

This rule can serve some purposes. If you want to call, you dont have to state that you are calling, or that you have to get some pocketchange from the dealer. You can just throw the chip in. I guess the rule serves more purposes in a casino game than in a home game, and can easily be removed.
You also have to say what you are doing, because the player that has to act after you can give you some information if you don't state your raise.
Example: blinds is 25/50, UTG throws in a 100 chip without saying anything. Then UTG+1 quickly says all-in.
This situation can only be there if there is any confusion around the rules, but still...