Freezing with flash
It’s not so much the shutter speed that freezes the motion here, it’s the flash duration. That’s because we’re in a dim room, so the only light that registers is the brief burst from the Speedlight, which is far quicker than the camera’s maximum flash sync speed (usually 1/200 or 1/250 sec). The lower the Speedlight power, the faster the flash duration. When fired at full power the duration may even be close to 1/200 sec, but at 1/16 power the duration will shrink to around 1/8000 sec or faster (depending on your Speedlight). So if you’re seeing motion blur in your flash-lit action shots lower the flash power and bump up the ISO.