I usually see it on the hand that doesn’t have the whistle in it. It looks like a little wristband connected to a strap that goes up the back of the hand and over one finger. I really can’t imagine what it’s for! Anyone?
There was a staff report and a thread about this.
http://boards.straightdope.com/sdmb/showthread.php?threadid=2411
The particular thing you mentioned is a strap that holds the official’s whistle. He moves the strap over each finger to keep track of the downs.