OK, the combo is wired towards the right side of the pocket so he would like to help the 9 ball go a little to the left. So he puts left english on the cue ball which imparts a bit right of english on the 6. This does three things that "make the pocket bigger". First, left english on the CB throws the 6 ball a tad right, so the combo is now aimed more left towards the pocket and, 2) Left on the CB puts a tad of right on the 6 which in turn throws the 9 ball a bit towards the left and, 3) Right english on the 6 puts a tiny bit of left english on the 9 ball so that if it hits the right side facing/point, the little bit of left spin will help it spin towards the hole - hence "making the pocket bigger".
BTW, the effect of #3, seeing that the 9 ball is the third ball in the chain, is negligible at best as there is very little (if any) gear effect remaining to impart left on the 9. But I threw it in there anyway since you never know if that little extra bit of spin can be the difference and it goes along with the effects of the first two items. Also, when you are not dealing with a combo this effect is a little bigger.
Hope this helps.