A semi-automatic handgun is a complex machine with multiple moving parts. It depends upon dozens of very small to large pieces of metal, along with several springs, pins, and explosive powder to function correctly. Sometimes, when one of these parts fails, the brass -- the part of the cartridge that holds the bullet -- will get caught halfway out of the chamber by the slide, instead of leaving the gun. When this happens the brass is left standing straight up in the ejection port, like an old-fashioned stove pipe.
Stove pipe Causes
Stove piping is caused by a variety of reasons. The bushings the slide moves over may be broken or misaligned, the grip of the shooter's hand may have slipped which causes the slide to not return properly or the brass may be deformed and not able to move where it should. A handgun that stove pipes continuously with a variety of ammunition is considered unreliable and should not be used.
Tap - Rack - Bang Method
An good way to clear the stove pipe is a the tap-rack-bang method. The shooter reaches with his free hand palm and taps the bottom of the magazine, ensuring that it is set properly into the handgun. He then racks the slide, or pulls it back. At this point some people roll the gun to the right, allowing gravity to help empty the chamber. Pulling the slide back allows a new cartridge to enter the chamber, allowing the handgun to go "bang" when you pull the trigger.
A different way to clear the malfunction is the slide method. When you bring the handgun up to the target you'll notice the stove pipe sitting in the chamber. Reach with your free hand, palm down, and sweep it from the front of the slide to the back part, knocking the stove pipe free from the chamber. Immediately rack the slide and the handgun is ready to fire again.
Simulating a Stove Pipe
Forcing the firearm to stovepipe in order to practice cleaning one is difficult. You can try limp wristing -- holding the handgun loosely in your grip when firing -- but that doesn't always work. Opening the chamber just enough to manually insert a brass mimics the effect of the stove pipe and allows practice clearing but it doesn't have the same effect as one happening in the process of firing through a magazine.