They set up barracades on the main roads, and jump out when people driving by, since if you hit them you will be in trouble, people would stop right down, then a bunch of rebels would surround your car to stop you moving away. They use prybars or steel clubs to hit your car's tires so you would have to either accpet to pay or being surround by angry mob that sure will hurt you if you don't obey.
Also do you even watch road condition in hongkong? We have narrow street that is maybe two lanes with both way traffic and it would be count as a main city street, most of the streets are one lane and one way in HK, if they block one side of them, there would be no escape, you can't back off because other car would behind you, you can't move forward because blockage, and you sure can't drive sideways because housing. You just don't able to move. We ain't your country road that you could drive everywhere to avoid danger, we don't have the luxery of room for that, almost all of the main city were in hillside or planned/created 100 years ago, think driving through old london's back alley, but with much more narrower and hilly path.