The crocodile tube wasn't common, but it was used to kill many infidels and thieves.
The victim was fixed inside a tube just big enough for the victim's entrance. The tube, having crocodile teeth-like spikes, was slowly compressed leaving the victim totally immobilized. The torturer could only see his face and feet.
With the help of carbon and fire underneath the tube, the torturer gradually heated the tube until he extracted a confession or killed the victim. The former was most common, as this is one of the cruelest and most painful tortures ever used on human beings.
With the face and feet exposed, the torturer was able to inflict painful wounds on the victim. Facial mutilation and toe ripping were preferred choices.