You have probably seen them on infomercials like this one:
These cookers look very appealing. After all, who wouldn't want to cook a large meal quickly, cheaply and with little clean-up. Unfortunately, there is a reason they are cheap: they are made cheap, which is a problem when you are dealing with a machine that uses highly pressurized steam at very high temperatures to cook food. Google TriStar Pressure Cooker consumer reviews, and the complaints online are abundant. Many claims and lawsuits against TriStar have been made against customers who have been injured due to the cookers exploding on them, sending scalding hot steam, food and water all over their body.
So far, the cases are very similar. The timer on the cooker goes off indicating the food is ready. The customer flips the release valve to let the pressurized steam exit the cooker and then, thinking all of the steam has been released, attempts to open the lid. Unbeknownst to the customer, there is still pressurized steam trapped inside the cooker. Once the customer touches the lid, the trapped pressurized steam blows the lid off the cooker and causes an explosion.
The cause for explosion is still under investigation, but this is problematic when considering the instructions for the product state extra safety measures are in place to completely prevent the lid from coming off if there is still pressure inside the cooker--these extra "safety measures" are clearly not operating correctly. Based on these instructions, a customer has no reason to think the lid will come off unless it is safe to do so.
In the older models especially, the problem seems to lie in the release valve mechanism, especially considering TriStar has upgraded its release valve mechanism for each new model that has come out. Because the valve is poorly made, food, debris and other particles get trapped in the valve, causing blockage that prevents all of the pressurized steam from releasing. It is not a coincidence that most injuries have occurred after the cooker has been used more than once.