xkcd.WTF!?

Explanation ➲

SawStop is an American table saw manufacturer whose products are designed to increase safety. Table saws are extremely dangerous tools due to their rapidly spinning circular saw blades protruding from the table surface. Accidental contact with the blade can result in severe injury or death. Even a stopped blade can pose a risk if the saw is not properly powered off and unplugged, or if the blade remains sharp.

SawStop saws feature an automatic brake that activates when the blade detects contact with human skin. According to the company, the system stops the blade within 5 milliseconds by releasing an aluminum brake block, after which the blade instantly retracts below the table. This mechanism is intended to prevent serious injury in the event of contact.

Here, Black Hat takes the concept of a safety-enhancing product and reverses it, creating one that is far more dangerous. His fictional invention mimics the SawStop's ability to detect skin contact, but instead of stopping the blade, it uses an explosive charge to start a stationary blade. This would be extremely hazardous, in triggering a rapid spin-up exactly when someone touches the blade. The comic suggests that it might even work when the tool is turned off and unplugged, which is typically considered a safe state. The explosive acceleration could cause severe injuries, and the device provides no functional advantage. Its only purpose is malicious, which is presumably why it's "less popular."

Explosive starters have historically been used in older piston aircraft and some tractors to rapidly start engines when electric starters weren’t practical. This method is still used in some modern aircraft, such as the B-52 bomber.

The title text references the fact that the SawStop brake mechanism is single-use. Once triggered, both the brake cartridge and the saw blade are damaged and must be replaced — though this is a small price to pay compared to losing a finger.^{[citation needed]} The fictional "SawStart" is also single-use, but in its case, the result is even worse: the blade becomes unstoppable, defying the second law of thermodynamics (and possibly several others). The comic suggests this could be the result of a collaboration between Black Hat and Beret Guy's mysterious business ventures. Since the spinning blade cannot be stopped, it must be replaced while still in motion — an obviously dangerous and impractical process. Even disposal would be hazardous, as the blade could destroy any container it's placed in.

A similar concept, subverting a safety feature, is explored in 2876: Range Safety, where a range safety officer is accompanied by a "range danger officer."