There are 3 US Handstrength recognized techniques used to close grippers. You could see one or all these techniques being used so be aware of the contest allowed set.
Mash Monster Set(MMS)
You use both hands to crank the gripper down until the handles are Parallel to eachother, then you start the close. The competition judge MUST see the set so make sure your free hand is out of the way when you start your close.
Credit Card Close(CCS)
BEFORE the start of a proper Credit Card Set Close, a credit card, or similarly sized card, MUST be PASSED THROUGH THE HANDLES. Do NOT hold the credit card over the handles and start the close or it could be disqualified.
No Set Close(NS/TNS)
A proper No Set Close is done completely with one hand. You are not allowed to use your free hand to "set" the gripper in a good position before the attempt. You simply pick up the gripper and close it.