Driving home today my trans got stuck in first gear after coming to a stop at an intersection.

I tried manual shifting, coasting in N and dropping back into gear, checked fluid,tried all I could it just wouldn't up shift. Reverse was fine.

I sat at the truck stop 30 miles from home stumped. It was then I remembered a like situation in my first car back in about 88.

I got onto the service road, brought my speed up to about 35mph, then slipped the shifter up into park for a couple seconds. Then back to drive. Bingo I was in third gear and had no more trouble the rest of the drive.

Question is what does this do? Back in 88 I did this at the recomendation of a trans tech. He said it causes a back flow condition in the valvebody and will clear a small partical or stuck check ball. Is this correct?

Other question is what got stuck? Trans is new but sat for around a year. Has maybe 300 miles use. This was first sign of trouble. Fluid is perfectly clean and on the full mark.