Why are Kakyoin and Polnareff so pleased about knowing the same hand signals? In episode 24 of JoJo's Bizarre Adventure: Stardust Crusaders, Polnareff takes advantage of a dangerous situation to show off his knowledge of hand signals. A video of this can be found here.

Kakyoin recognises the hand signal as meaning "Your underwear is showing", which greatly pleases Polnareff. The two of them share some kind of elaborate handshake to mark the occasion, the eyes of both characters being darkened (which, as far as I understand, usually indicates seriousness).

What on Earth is going on here? Why do Kakyoin and Polnareff know this, why is Polnareff so pleased and why do they share an elaborate hand shake afterwards?
Polnareff knows this by virtue of him being Polnareff.

Combining the individual gestures and their 'translations' makes the sentence that it does.

Pan (Japanese sound for clapping) + 2 (two) + ok (maru) + seeing something (mieru) = pan-tsu (underwear) + maru (entire) + mieru (can see)

Only Kakyoin is able to decipher what he's trying to say, since NO ONE ELSE chimes in, and they both share a bonding moment. That bonding moment is the amazing handshake, and their eyes darken to reflect the solemnity of the bro-ment.