He explained quite astutely in a you tube that once released, the cue can only continue on that line.I was talking with a few Taiwanese players about this a while back, among them Big and middle Ko (who also do this).
It's using natural impetus to ensure the line of the cue through the ball, when a firmer grip can potentially cause more deviation against the edge of the rail. I think it's a 'feel' based rule when applying it. I find it works well for me, and I am not at a level to critique other players who I know employ this method.