Sometimes they run the test by picking "random" people out of line. If you time it right you could avoid it that way.

There's not really a good way to avoid a false positive. The test alarms on at least these things:

1. Explosives (duh)
2. Explosive powder residue (avoid the firing range)
3. Fertilizer (avoid gardening)
4. Certain hand lotions
5. Nitroglycerin heart medication
6. The leftover residue from the last person who used the same swab
7. Contaminant on the TSO's gloves
8. Potentially, the machine itself could be contaminated and/or not recently calibrated
9. Surprisingly often, for no reason in particular

Keep in mind, the test isn't directly for explosives. It's for a class of chemicals that are components of explosive compounds and also components of many other things... So aside from the contamination and calibration issues - and any unexplained false positives - the test works as designed.

That's why a positive on the test leads to a pat down. The pat down is supposed to clear the alarm by "proving" there are no actual explosives on the person.

One of the problems is that many TSOs confuse a positive test with a confirmation of explosives. That was apparently never the purpose of the test. It was supposedly designed as a first alert that there may be something dangerous happening.

Yeah, that's not a very good design... but there's supposedly nothing better available.