Watch this video of Efren playing. Notice at 2:00 - 2:06 and see what he does with his back hand.
This is not a "slip stroke" like Cornbread used to do, however, it accomplishes many of the same things......many players do this, it's just not detectable until you are made aware of it.
LINK TO EFREN REYES VIDEO - NOTICE 2:00 - 2:06
It looked to me like he was in a rhythm, a quick pace, and was simply combining the tasks of positioning the cue in his grip and doing some practice strokes. It did not appear on his final practice stroke that we could see that the cue moved in his grip.
I would not consider what it shown here as "part" of the stroke.
KMRUNOUT