Since 2014, London has faced a surge in crime committed on two wheels — so-called ride-by theft in particular. Riders often take advantage of people distracted by their cellphones as they walk down busy sidewalks, snatching the devices from their hands, sometimes violently, and grabbing up to 30 an hour, according to the police.

Others snatch women’s purses or assault people as they ride by. They target crowded streets and escape through alleyways, sometimes even discarding their helmets in the belief that it will make the police more cautious in chasing them.

The police call the practice of deliberately running a vehicle into a fleeing motorbike “tactical contact,” and they say that only specially trained drivers do it. According to The Guardian, the tactic was deployed 63 times this year, and three incidents have been referred to the police watchdog for investigation.

One qualified driver, Sgt. Tony McGovern, told The Guardian last month that tactical contact was one of the hardest to use: “It’s just a slight nudge. It’s controlled.” He said suspects were amazed when, in some cases, they were sent sprawling off their bikes.

“They are shocked,” he is quoted as saying. “They say, ‘We did not think you were allowed to do that,’ especially when they take off their helmet. They are confused.”