This is a direct email I got from the PBS guys about this exact question.

"What is it:



The credit push is defined by any line holding pilot who is awarded more than the minimum credit (usually 75 hrs) without asking for it in some way in their bid. It happens when there is more credit in a base then 75 hrs (usually) per line holding pilot.

For example, if preassigned credit + trip credit / total number of pilots in a base worked out perfectly everyone would get 75 hours of credit and get 16 days off.



When does it happen:



With 500 hrs vacation and training in a month

With 7000 hrs of trips in a base in a month

With 100 pilots holding a line in that base in a month

If the fake numbers above work out, those 100 line holders would get 75h00 of credit for that month. If you take more credit then this in the base pilots will start to be pushed.



Where it happens:



It happens at the bottom of the line holders. Since 7500 hours of credit in a 100 pilot base usually isn’t the case, we have to cover that additional flying. Here is a screen shot from last month of the bottom 35 line holders with a few people who bid reserve in there as well. Line type is the first column, next column is credit for the month, followed by the days off column. The ones that say REGULAR are the regular line holders, the ones that say P2 are reserve, so skip them. Work your way down the list, the first group is between 75-80 hours, those aren’t credit pushed. Then there is an 85h37 credit hour line. Following that there are more 75-80 line credits. That would mean the 85h37 hour line bid for credit in some way, so he isn’t pushed. Fast forward to the bottom. You’ll notice a huge increase in credit awarded. 93, 86, 81, (skip the reserve), 87, 81, 89. Those numbers are way higher than the 75 hr minimum the solver tries to give each pilot, but the flying has to be covered. So it (credit) pushes that flying on to those pilots. The pushed pilots usually end up with more credit, less days off, less bid preferences honored."