Hi,

I heard that after shaking up a can of soft drink (coke for example),
you can tap the top of the can and it will not fizz up when opened...

The answer i am looking for is simple, just true or false and some
description of the process involved if true (depending on the answer).

Hi! Thanks for a very interesting question. 

It seems that the resources show that tapping may not be enough to
stop a shaken soda can from exploding. The element of time of settling
the pressure inside the can is still the bigger factor.

"Tapping on the top of a carbonated beverage can (or bottle) before
opening it may help prevent the drink from exploding out of the can
upon opening (although it might not be sufficient!)."

"If the can is opened immediately after shaking, it is likely that the
beverage will spray all over the place, since some liquid from all
parts of the can will be pushed ahead of escaping gas and out the can
(hey, bubbles float to the top and these bubbles are under pressure
too!). When the can is tapped before opening, bubbles adhering to the
sides and bottom of the can may be jarred free. Assuming the can is
upright, the bubbles, being lighter than the liquid, will float to the
top of the can."

"Why do people tap on cans?"
http://chemistry.about.com/library/weekly/blcantap.htm

"Cans of soda have lots of carbon dioxide trapped inside at very high
pressure. The fizz in soda is the dissolved carbon dioxide gas coming
out of the liquid, bubbling back out into the air. The high pressure
in the can traps the gas in the liquid so when you open the can and
lower the pressure (That's the hiss you hear when you open a can: high
pressure carbon dioxide leaving the can.) the gas starts slowly
bubbling out of the liquid and tickling your nose.

"If you shake a can a lot, or drop it, the gas trapped in the liquid
temporarily comes out of into the rest of the space in the can. More
gas is then trapped in that small space at higher pressure than
usual."

"Why doesn't a can explode when you shake or drop it and what happens
if you tap it afterwards? Because the can is very strong due to its'
shape. The circular symetry of the can gives the aluminum great
ability to contain the pressure inside. Tapping it just pushes a
little more gas out of the liquid, but won't dramatically change it.
Tapping a can that's already been shaken actually forces more gas out
of the liquid and ends up raising the pressure in the can."

"Why can a soda be shaken or dropped and then if I tap it it won't
explode?"
http://web.archive.org/web/20010414033554/www.omsi.edu/explore/whatzit/

"No matter. First let's consider the matter from a theoretical
perspective. Carbonation is produced by forcing carbon dioxide into
solution with H2O under pressure. Shake up the can and you create
thousands of micro-size bubbles. Each bubble offers a tiny surface
where CO2 can rapidly come out of solution, creating the potential for
explosive fizzing should you open the can prematurely. Wait a while
though, and the bubbles will float to the top of the can and
disappear, and eventually all will be as before."

"But suppose you're the impatient type. You tap the can. What, pray
tell, is this supposed to accomplish? Are we going to noodge the tiny
bubbles to the surface faster, after the manner of herding cows? Are
we going to maybe dislodge a few bubbles that have stuck to the sides
of the can? Maybe we are, but the difference is slight. Open that baby
and you're still going to get a faceful of froth."

"Will tapping a pop can keep the carbonation from exploding on
opening? Plus: why do we tap cigarettes?"
http://www.straightdope.com/classics/a4_198.html

Our last link meanwhile says that it could be done if you use a metal
object to tap on the can.

"If the can is tapped with a metal object the vibration releases the
bubbles and they move to the top of the can where they join up to form
bigger bubbles or mix with the gas at the top of the can. If you then
open the can, the gas escapes without dragging the liquid out of the
can with it and onto the unfortunate bystanders. If the can is not
tapped, the bubbles expand when the pressure is released and push the
liquid out ahead of them."

"Spray can"
http://www.newscientist.com/lastword/article.jsp?id=lw366

Search terms used:   
tapping soda can carbonation exploding 
       
I hope these links would help you in your research once more. Before 
rating this answer, please ask for a clarification if you have a
question or if you would need further information.
       
Thanks for visiting us again.        
       
Regards,        
Easterangel-ga        
Google Answers Researcher

That was far more information than we expected and are very happy with
that answer! 5 stars for effort!

at last! the mystery is solved!

I thought the idea that tapping would cause bubbles stuck to the
bottom and sides of the can to break loose and rise to the top where
they would not propel the soda out of the opening, most believable. 
Likely part of the improvement, if any, is due to the delay in opening
the can while you are tapping. If you tap vigorously, the spraying of
the soda will (all but surely) be worse.
 To avoid confusion, the fissing and shooting from the can should not
be called explode. The can is made strong enough to resist explosion
up to about 140 degree f, but you should retreat promptly from a can
of hot soda as it could throw shrapenel.  Neil