From what I understand, when in Av mode, the flash behaves largely as fill flash - the exposure is more or less set as if no flash is in use. However, in Tv mode, the manual suggests the shutter speed will be honored as along as it is less than flash sync speed.
However, in my test, this is not the case (Canon 6D + 600ex): In Tv mode, I am seeing similar behavior as in Av mode. The actual shutter speed of 1/6 or 1/10 is not what I set at all (1/100). I am confused ... what am I missing here?