I wouldnt think that colly would use a heavy bat to make his match bat feel light.
There was a similar idea in golf with a practice club that was twice as heavy, the idea was that it would be easier to keep the club on line.
I think the idea with the bat is that id would make it easier to get your hands through the ball, improving driving especially. It makes sense as if anyones ever used a really light bat theyll know its harder to keep the bat on line. Using the heavy one should ingrain the correct feeling!