Whenever you cast an instant or sorcery spell, flip a coin. If you lose the flip, return that spell to its owner's hand. If you win the flip, copy that spell, and you may choose new targets for the copy.
With Sakashima removing he legend rule (and likely entering as a copy of Krark) it gets complicated. The question is what happens to the other Krark triggers when you lose a flip? Do they still resolve and copy the spell, or do they fizzle with the spell no longer on the stack?