This just means that if there is a problem for the first match of a set on the order of choosing characters then both players can either play rock paper scissors to decide on who chooses their character first, OR they can call a TO over to whisper the characters to the TO and then MUST play with the character they said they would go (regardless of the opposing player's choice of character).
EX:
FURY mains Lucario, Falco, and can also play MK.
SMURF mains Pit, Diddy, Snake, and can also play MK.
If Fury picks Lucario first, Smurf will pick Diddy. If Smurf picks Diddy first, Fury will pick MK or Falco. So, in this scenario the player who chooses last gets the advantage. To avoid that, we call over Nasty.
Fury whispers Falco into Nasty's ear (no homo, lol) and Smurf whispers Pit into Nasty's ear (homo, lol) and then they must play the first match of the set with those characters.
In the end, Fury's Falco 2-stocks Smurf's Pit and Fury goes on to win the set.
If you need further clarification see Smurf's side of the story (which I'm sure he'll post after hearing my analysis of this hypothetical outcome).