I am reading Hunter x Hunter, and the two protagonists (friends, and kids) are given cards that they are not supposed to reveal for the duration of the game. They start talking and they exchange a bit of information and then this is said :
Then, they proceed to show their cards at the same time (saying せーの).
I understand that one asks the other to show the card but I am unsure as to what is this form (is it a contraction of sorts?). Does it comes with a particular nuance?