Alas - not me...........
Well, it was 68 years ago and I'm a just mere youngster
Using wikipedia https://en.wikipedia.org/wiki/Rollie_Free
Roland was born in 1900, and in 1948 on a Vincent HRD owned by John Edgar, he rode in the now-famous prone-position to cut down on wind resistance, clocking a very respectable 150.313 mph.
I attended a UK Motorcycle Sprint meeting a few years ago and was witness to one of the more rotund riders (in full race eathers, lid etc) mimic the prone riding position, but on a rather sedate 250cc AJS with mudguards etc, it somehow didn't quite do it............ (but we did split our sides laughing!!)
As for the gearchanging, he would have been in top gear before getting prone, and with a mile or two run up, no real panic.
For those that have watched "The World's Fastest Indian", William Lucking plays Rollie Free, introducing himself to Bert towards the end of Bert's Tech Inspection & nearly getting the oily handshake, and features a fair bit during the Bonneville sequences.