Brian Krebs, who has written many excellent investigative pieces on ATM skimmers, spent several hours watching footage seized from hidden skimmer cameras, and has concluded that covering your hand while you enter your PIN really works in many cases -- and that many people don't bother to take this elementary step.
Some readers may thinking, “Wait a minute: Isn’t it more difficult to use both hands when you’re withdrawing cash from a drive-thru ATM while seated in your car?” Maybe. You might think, then, that it would be more common to see regular walk-up ATM users observing this simple security practice. But that’s not what I found after watching 90 minutes of footage from another ATM scam that was recently shared by a law enforcement source. In this attack, the fraudster installed an all-in-one skimmer, and none of the 19 customers caught on camera before the scheme was foiled made any effort to shield the PIN pad.
Krebs goes on to note that this doesn't work in instances where the skimmer includes a compromised PIN pad, and it seems likely that if covering PINs became more routine that crooks would take up this technique more broadly. But for now, covering your PIN with your free hand is a free, effective means of protecting yourself from ATM skimmers.