As Montreal police are beginning to increase the use of kettling in their crackdown on protest, it's a good time to remind ourselves of the dual use of that tactic: 1) the production of crowd "violence" providing pretext for further crackdowns, and 2) as a form of extra-judicial punishment (I am choosing that term carefully; I do not mean "extra-legal" but simply "outside the courts" [as opposed to the legislature]).
The best new crowd psychology shows that the police can produce violent reactions from their adversaries, just by treating them as adversaries (thereby producing a group identity of the crowd as "enclosed" and "threatened"), cutting off their path ("kettling"), hell, just showing up in riot gear. This effect is not unknown in the policing community (paywall protected article, unfortunately). You can of course investigate the self-constition of police group identity as the "thin blue line protecting society from violent mobs," etc. along these same lines.
Furthermore, as Laurie Penny points out here, kettling itself is a form of extra-judicial punishment: humiliating, anxiety-producing, etc. Just keeping you from urinating for hours has all sorts of subtle or not so subtle physiologico-psychological anxiety effects. It also puts the relief of your anxiety in the hands of the agents of the State.
One other extra-judicial punishment, beyond kettling -- or maybe a prolongation of kettling is better -- is to make the arrest procedure as long and degrading and difficult as possible. And then to decline to press charges. But the 24-36-48-even sometimes 72 hours in custody is in fact worse punishment than a judge is likely to pass down. So even if the police don't break your ribs with a billy club or pepper-spray you, they can still punish you, for just keeping you handcuffed, humiliated, for hours and hours on end is already punishment.