Don't be afraid to bend your knees during the kettlebell swing!
The first swing has a more athletic hinge...more knee bend, deeper hinge.
The second a more RDL-type hinge.
Here's a screenshot so you can see those two different hinges better:
Even though you'll feel those hammies with the RDL, you'll want to be careful not to pull from the low back on the backswing.
The athletic hinge with more knee bend is going to give you more power and you'll use your legs/hips more. I also feel it's easier on the back.
Go deeper and try the more athletic hinge the next time you swing and let me know if you can feel the difference!