This signal is used by the Referee to signal that a player is wearing illegal equipment or that his equipment is being used incorrectly (eg chinstrap undone).
This is a violation and usually results in the offending team losing a time-out to enable the equipment to be made legal. If a team has no time-outs remaining then it is treated as a Delay of Game (S21).
There is an extreme case of a player wearing illegal cleats, where the offender is also disqualified (S47).
Begin with both hands held by your side and with an unhurried motion bring one of them up until it touches the back of your neck.
Finally point to the offending team by stretching an arm out from the shoulder to the horizontal.