Hi, why are japanese people compere each other hands?

Because I worked today at the world tournaments swimming for handicapped people and I am at the doping control and we pick random people and lucky me i had a japanese guy today!! So i had to follow him till he pied (we cant choice them by ourself)Later after some good laughs the swimmer was comparing my hands with his own hands, thumps on thumps and hands on hands he also gave me a ?crossed handed shake?.

What do they mean with this?

Also asked their bloodtypes and one swimmer in the team had also AB like me so that was funny.

Made some photo's with them to and they will send it me when they are back in Japan!!!