There are a lot of pieces to the police strategy, and one key is to keep the crowd moving. It is an expensive lesson that Chicago learned decades ago that you do not kettle the crowd, or leave the crowd without a visible means of exit.
Many of the marchers Saturday night were not sure where they were going, and the police were constantly trying to steer them.
The crowd, for instance, came to Washington and Dearborn. The police set up a bicycle blockade. The crowd pushed. The police commander made the decision: let them through.
Some in the crowd cheered, claiming a victory. The reality was the police decided to release a pressure valve and keep the crowd moving.
The crowd goes one block and comes face-to-face with another bike line on State Street. This time, an officer is pushed; a billy club is swung, and police shout the command: "Hold. Hold."
A police officer goes down, as do some protestors. Then come the arrests.
Every time a line is established it's to direct the crowd, and/or to position police so that they are ready once the line is opened.
"We want to steer them out of places where it's going to be problematic and facilitate them moving, that's all, and sometimes it takes a little while, but by and large, I think - not so bad, the cops are doing an amazing job, that's all I can tell you."
There's been constant negotiation between police and crowd leaders, which hasn't always been easy cause its often not clear who the leaders are.
"That is simple crowd control. If they box people in and people feel threatened like there is no escape and panic starts, it can be a very dangerous situation; it has nothing to do with public safety," said a man who identifies himself as Vermin Supreme. He is a veteran of many protests, and he says that, all in all, the police have been very restrained, which is a huge part of the strategy: a restrained response. The police Saturday night had on helmets and face shields, but not the heavy body armor or turtle suits that can be perceived as provocative.
It is likely no secret to the protesters that as they walk, there are among them some plainclothes police officers, who are somehow able to take the temperature of what is happening and perhaps learn what movements may be forthcoming.