It's when you normally feed 4-5 pounds per days, but 2 weeks (or 1 week) before your projected date for breeding you start feeding like 6-8 pounds/day. Make them think things are more than good and their brain goes nuts and brings them right into cycle like they should.
We usually go with 14 days flush feed coinciding with 14 days of matrix and move them into our barn (change in environment) around day 12 of matrix so we are tripling up our chances of getting our sows/gilts to cycle and when we want them to and then we also followup with PG600 on day 14 or 15 (depending on delivery dates). It has worked with all of our gilts except 2 over 3 years (6-8 gilts/sow herd per year). We do not have very many litters/year so we can't afford a screwup on our part.
Looking back our 1 sow was probably standing as we started matrix so it didn't work and she came in 21 days after starting matrix--AI'd on day 9/10 with semen...didn't take. Our second gilt just did not breed after 3 very closely timed cycle history/behavior--she is no longer with us.
"Your present circumstances don't determine where you can go; they merely determine where you start."--I Am Second.