Scottie Scheffler used at least some of his time in jail on Friday to prep for the PGA Championship ... admitting he actually got some stretching in while he was locked up.

The 27-year-old just addressed his arrest following the conclusion of his second round at Valhalla Golf Club in Louisville, Ky. ... and he was frank with the gaggle of reporters who had gathered to talk about his early-morning run-in with cops.

He said repeatedly he was very confused as to what was going on during the encounter ... and he said, candidly, it all really rattled him.

But, somehow, he told the media members that while he was behind bars ... he had the wherewithal to continue his warmups for the big day -- even though he wasn't sure if he'd get let out in time to tee off.

Turns out, it might have helped him big time ... because he went on to shoot a 5-under 66 -- putting him just a couple strokes out of the lead as the tournament heads into the weekend.

Despite the score, though, Scheffler did say his head was "still spinning" over how everything had gone down in his leadup to the first tee box.

Scheffler -- who's now facing four criminal charges over the incident -- won't have much time to digest everything from here ... he's got to be back out on the course Saturday morning.

But, maybe some calisthenics in some tight spaces will now be in the routine going forward?