1 year old I baby sit throws himself on the floor, WTH?

That's completely normal behavior, especially for little ones that don't have the words to express what they're feeling. The best way to stop a tantrum is to prevent it. Figure out what triggers the tantrums and then you can figure out how to prevent them.

for example, my 13 month old cries, screams, and slides herself to the floor for a tantrum if she's told no, or that she can't have or can't do something. Since she doesn't hurt herself, we usually ignore her. But, it also helps to explain quickly, and with simple words, why they can't have or can't do what they want at that moment. It also helps to give a short time out in the crib until they can calm down.