There's a lot of misunderstanding about prop swinging. But both the swinger (who is the guy in charge at all times) and the cockpit occupant (who follows the swinger's instructions at all times) need to be fully and correctly trained. It's not at all difficult, but there's stuff you MUST get right or it can be dangerous.
And the engine doesn't burst into life immediatly, threatening to chop off your hand. As you swing you step away, the engine 'catches', continues to rotate, and it's a couple of seconds before all cylinders join in and it's up to tickover speed. By then the swinger is (or should be) quite a long way away from the prop arc.
Minor injuries are sometimes caused by the engine firing before top dead centre and flicking backwards while the swinger's hand is still on the prop. Ensuring only the mag with the impulse on it is switched on will prevent this, and not curling your fingers around the prop's trailing edge will ensure that even if it does happen, no injuries should result.