I've had it done where they rub it with a cotton ball and then place the cotton ball in a machine that detects explosive traces. I think this is the swabbing test. Quick and painless.

What bothers me though is when screeners for no apparent reason manipulate my electronics without asking. Case in point: CVG I was bent over putting my shoes back on (no lane at the checkpoint). My laptop had cleared the machine and was still in the tray. My laptop bag was next to it. A screener (I'm assuming because it was a slow day) came up long after I had had a chance to leave (I was only there because I had to re-tie my shoes) and opened my laptop without asking (was she looking for a knife between the screen and keyboard?) then immediately shut it again. She didn't slam it closed or anything improper, but in the process of opening it, it came out of hibernate mode; reshutting it so fast kind of messed things up a bit.

Had she asked, I would have had her open it up and then return it to me so I could shut it down properly. Now I keep a much better eye on my gear.