You’re sitting down and interviewing (or simply speaking with) a somewhat older and certainly more famous fellow than yourself, and as the conversation is winding down he affectionately, quickly, semi-aggressively grips your knee.
That’s a gesture of courtly approval — it means that you’ve passed inspection.
I don’t know how many times this has happened to me personally, but I’d say a few. I’m thinking in particular of a 1999 Toronto Film Festival party for The Limey, and hanging for a half-hour or so with the great Terence Stamp. As the party was ending and we were all starting to disengage, Stamp gave me a nice fatherly knee-grab — not too gentle, not too aggressive, right in the middle.
I can’t honestly say I’ve ever knee-gripped some younger guy. I tend to prefer shoulder grips or upper back pats.