Stop Stick is a trademarked name for one of the more popular 'Tire Deflation Systems' used by police and other government agencies. The term has become so common that media outlets may use the term for other tire deflation devices, and currently Stop Stick has nearly cornered the US market. Stop Sticks are best known for being used by the police in car chases; the Stop Stick is the default option for intercepting a fleeing vehicle, as they are designed to deflate tires at a controlled rate, stopping the car safely. Some models are used as mobile STDs at traffic stops and in front of tires of cars at crime scenes to prevent the chase from ever starting.
Stop Sticks are essentially modern-day caltrops, consisting of a series of hollow, pointed tubes mounted on a lightwight stip. The hollow quills remain in the car tires, releasing air at a controlled rate and preventing a sudden blowout. The strips may be laid across the road manually, or they can be thrown onto the median and pulled quickly in front of the target vehicle by way of a retractable cord; in the case of a car chase, the retractable cord method is most common, as it prevents other vehicles that might be on the road from running over the Stop Stick, and allows it to be quickly removed so that pursuing police cars don't run over it.
There are a number of different models: the basic model is simply called the Stop Stick; the Barracuda is a heavy duty model for trucks and other large vehicles; the Terminator is a small, easily portable model; the Piranha is a single small triangular wedge for placing directly in front of a tire. These small wedges are in fact the basic unit of every model, an acute triangle with the sharpest angle pointing upwards, which houses the quills. Once a quill penetrates the tire, that segment of the strip must be replaced with a new cartridge, bought from the manufacturer.
A determined miscreant can keep driving even with flat tires, but at a reduced rate and with reduced control of the vehicle. If nothing else, a 40 mile-per-hour car chase is less dangerous to bystanders (and those directly involved, of course) than a 100 mile-per-hour one.
You can learn more at stopstick.com. In the interest of unbiased reporting, it appears that there are some competitors, the Stinger and the Magnum Spike that are also making headway in the US market.