the procedure I was taught back in 1978 or so... the old tranny geezer told me to pop the tranny into reverse at a low speed and stab the throttle for a second in this 68 Mustang with a C4.
The tranny started shifting great after I performed the reverse trick
After I got back to the shop, the geezer asked " was that you squeeling tires?" I proudly said 'yes' since I thought I had done the procedure correctly.
He then explained how I should have done it in the alley on the dirt , not the asphault and not smoking the tires.
The tranny guy had to be at least 50
Yesterday is history.