Three shots: Obviously this example is not ideal for fill flash, I just wanted to demonstrate how the DOF can be limited and still have use of a flash.
1/1250 f4.5 no flash for reference of DOF.
1/500 f9 TTLBL fill flash
1/1250 f4.5 (sync override using tape as discussed earlier) manual fill using 1/64th full flash...kinda hot but notice DOF compared to using the limited sync max of 1/500
One thing is for sure, there is plenty of light coming out of the flash.