I thought it worked as you put money from the transfer budget back into the main bank account (as it were) so you got higher interest coming in. Could be wrong.
I dont know how it works (think its something to do with higher interest rates) but if you move the slider all the way to the left to show as much wages as you can then your club will earn more money. I always do this inbetween transfer windows anyway as I'm not going to use the money and its probably going to benefit me in the long run.