They'd pinch themselves to see if they're awake and then quickly score before the defending team recovers from their madness.
You really think so?
Lets say the opponent takes a corner from the side of our left back.
- Our left back would be there to prevent an easy short corner.
- Our two central defenders would be in the box, as well as our right back.
- Our CDM would be around the edge of the penalty area.
That means a short corner wont be easy and there would still be 4 players around the box to challenge a cross.
But more important. How many players would the opponent bring forward, when we still have around 5 players (3 attackers, 2 midfielders further on the pitch) ready to strike on the counter? Additionally the goalkeeper would have a lot more space in the box to move around towards the ball if its being crossed to be able to catch it. Generally every extra player we bring back allows the opponent to bring an extra player to bring forward, making it harder for our goalkeeper to be dominant..
I've never ever seen any team do this (perhaps for a reason), so I really have no clue how this would turn out. I really think the opponent will be very wary to bring many people forward.