I ran across this same topic at Last Word, Why do cans of drink not spray if you tap them first?. They compared it to winding babies. Could there be a difference between using your finger and using a spoon or key?
They are wrong. Why would a key or spoon do differently than with your finger? Are you proposing there’s some sort of magnetic effect involved?
But I have no idea what “winding babies” is. Unless that’s referring to colic? That is not the same thing. Colicky babies have their stomach valve in the way (can’t remember what it’s called - not epiglottis, but…). Patting them on the back helps them burp. Not the same thing as knocking dissolved gases out of solution or making them reabsorb.
Clearly the Last Word article is referring to “burping babies”, in which case, yes, patting them on the back does work, to get bubbles ingested with the milk up and out. But you’re right, colic is in the intestines and patting the baby doesn’t help.
A metallic object would make a sharper tap than a soft fingertip. Maybe that makes a difference? We need somebody to go get some pop cans and experiment.
I think we have some drink cans at home, a coupla Mellow Yellows. I’ll check it out.
(You from Minnesotta, DDG?)
Land O’ Lincoln gal, born and bred. “Pop” cans. 
And that’s as in northern Illinois. I believe in southern Illinois they’re “soda” cans. A quick consultation with La Principessa packing her lunch out in the kitchen reveals that here in Central Illinois her classmates in the local fifth grade refer to them about equally as “pop” cans and “soda” cans.
Linguists make reference to something called the Bloomington-Dixon isogloss, an imaginary line that divides Northern Illinois speakers from Southern Illinois speakers, and that runs through (you guessed it) Bloomington and Dixon.
By Abraham, you’re right:
http://www.ugcs.caltech.edu/~almccon/pop_soda/
I never noticed that division through Illinois before.
Pennsylvania and New York have an east-west divide,
and the Carolinas are just messy, but otherwise
each state seems to be pretty much homogenous.
I came across this site the other day http://www.quizland.com/popquiz.mv , that states that tapping a can with a key, ring, or other metal object will make the fizz go down. I sent an email asking for info to back that up, but haven’t gotten a response. So if you take this quiz, the answer to the last question is “True” if you want the points.
For what it’s worth, I have a freshman-level physics book which claims that this works, supposedly by dislodging bubbles. On the other hand, this same text claims in the previous sentence that the pressure is not any higher in a shaken bottle than an unshaken one, which is provably false.
I’ve always opened cans just a crack to release the pressure, then open them all the way. The only time I’ve ever had an ‘explosion’ doing this is when the can came from my brother after it tumbled down the stairs (and after he gave it a few good shakes, to be sure). Otherwise the worse I get is a quick mist if it is only slightly shaken, but no foam-over or eruption.
Regarding the aforementioned pop/soda debate see the current thread:
http://boards.straightdope.com/sdmb/showthread.php?threadid=73166