For me, I went to a deep knee bend for two reasons:
1) I hit up through the ball, and doing so from a more upright position caused me to loft the ball to the arrow on the fly. I decided that to get more roll while keeping the same release would require me to finish lower so that the ball hit the lane sooner. I already had a knee bend, but I made it much lower.
2) Related to this, being lower made it much easier for me to get the leverage and angle I needed to lift up through the ball. Personally, I can't lift up through the ball when I'm bent over at the waist with little or no knee bend.
All that being said, with the new rage being the "open handed" or "don't hit up through the ball" release, a deeper knee bend probably isn't as necessary now -- at least not for lifting through the ball.
I think we call all agree that it's important to be balanced at the line, whether you're straight-legged or with knees bent.
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You weren't unlucky when you left that corner pin, so shut up about it already!!