A handshake in which a person reaches out for a handshake with someone they had just met but quickly fakes him/her out and goes for a hug but then again fakes him/her out by stretching his/her arms out in what appears to be a falling airplane position and quickly grabbing/fingering his/her balls/pussy.
Hence the "crash landing".
Joe: Yo dan why does it look like you've just seen your grandma getting fucked in the ass?
Dan: dude some guy just went to shake my hand and hug me at the same time, but instead he grabbed my fuckin sack!!
Joe: HAAA! sounds like you just got a crash landing!