Another suggested technique… following up from yesterday.
Try this: have people throw something during the shot.
Your flash takes around 1/1000th second at full power. So whatever is thrown will be frozen in mid air. Like this piece of clothing:
Again, as in yesterday’s shot, you will need to pre-focus, hold the shutter half way down, and then press when the item is where you pre-focused. A little tricky but worth it when you get unusual photos as a result.
For this to work, make sure your camera only sees the flash light. Manual exposure mode, 400 ISO, 1/125th second, f/5.6 should do it indoors. Test this by shooting with the flash turned off: your photo should be black.