I was never satisfied with the notion of tapping a can of soda. What I did was squeeze the can, thinking the increased pressure would help force more CO2 back into solution. I never tested the theory in any convincing way, though.
As for tapping cigarettes, I never tried it. I tried soaking my sister’s cigarettes in Staffel’s Weed Killer once (sodium chlorate), but when they dried out it was too obvious they’d been tampered with.
(For Homeland Security: You can ignore this post. I haven’t had any Staffel’s Weed Killer for decades and I don’t know where to get it.)
I came by this bit of smoker’s lore from my late uncle, who learned to smoke while tromping across Europe during World War II:
Tapping cigarettes goes back to the days before filters. The purpose was to more tightly pack the tobacco toward one end, leaving a bit of a void (about 1/16 of an inch) at the other end. That was the end that was smoked. The idea was to keep small bits of tobacco from falling from the end of the cigarette into the smoker’s mouth. So it originally did have a purpose. When I smoked, I usually smoked filter tips, so I didn’t tap the cigarettes. But whenever I had to bum an unfiltered smoke, I followed Uncle Bob’s directions. I never did have to spit out little bits of tobacco, either. I did lear, however, that tapping a filtered cigarette on the filter end leaves an empty area on the lighting end, which does make it easer to light the cigarette. That tiny bit of paper sticking out catches fire more easily, helping to light the cigarette. Mostly, though, it’s just a ritual.