What did people say when snatching something before the word yoink was invented? "Ah ha - I have taken it from you!"? No, that just doesn't work. Since the time when one caveman first stole a leg of mammoth from another caveman, there has been a yawning gap in our language - an exclamation to be used when lightheartedly taking something from under the nose of its legitimate owner.
The key to the proper use of yoink is a lighthearted or impish attitude on the part of the thief in the quick removal of a physical object- George W. Bush, for example, could not have said "Yoink!" upon his inauguration, whereas Robin Hood would have said it regularly.