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
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.