Some movements seem so simple we hardly think about them. They're so easy that they're often use in high reps. One of these moves is the kettlebell swing. Here are the big three problems I see regularly:
1) It's a hip hinge.
And by that I mean that it's not a squat.
The kettlebell swing most closely resembles the deadlift in the shape you create with your body. Your shins stay vertical, your hips push back, and your back inclines accordingly. The difference is that you're explosively pushing into the ground with a weight near your hips, causing it to be flung forward.
I could rant about the bent arm kettlebell swing and how what you're really wanting to train is the kettlebell snatch, but that's a chip on my shoulder for another day.
2) This isn't a club, don't booty pop.
Where your lumbar spine connects to the sacrum isn't the sort of joint meant to move under load. So often I see people let their low backs go either during the back swing (usually because they want to keep their chest up) or after they pop the kettlebell forward because they aren't maintaining tension in their tush. That excess movement is going to make your back mad, mad, mad.
3) Stay within your shoulder range.
We have a love for the overhead swing in CrossFit. But just because Games competition athletes are expected to have their shoulders reach 180 degrees of flexion, you don't need to force it. Face it, most of us have been hunching over a desk since kindergarden, and our shoulders aren't ready for that. Only swing the kettlebell as high as you can while maintaining tension in your butt and abs.