The term is "kaiawase" (貝合わせ) which literally means "joining (matching) shells together" but in slang it translates to act of lesbian sex (tribadism to be exact).
It's not the first time in anime when two girls are innocently playing with shell matching anyway... :)
TomoyoViBritannia said: Is this the original of the pic? They're holding hands? Does that mean NanoFate is finally canon? I'm going to be pissed if this is a shoop.
Two girls holding hands does not a yuri couple make. On the other hand, this is hardly the first official art that hints at that sort of thing.
Don't worry Tomoyo, this couple doesn't depend of bad shooped fanart to be canon. You should re the new Nanoha Vivid manga if you want canon NanoFate, Nanoha Force is another new manga but Nanoha and Fate barely appear.