Right before I pull. Look at some videos on my training log, I grab the bar and do a couple of hip pumps and before I do the last one and pull I take a breath and brace harder. There are different ways to do it, this is just what I have found works best for me. I used to just kind of sit in the bottom position for a couple seconds while I set my grip, took a breath, and braced, but for some reason the hip pumping thing makes it feel better.
Are you using a stiff bar or DL bar? Before I started using a DL bar it never felt like there was any slack to pull out, switching to a DL bar really threw me off at first. I think you have to pull like 700+ on a stiff bar for there to be any slack worth thinking about.