Mechanics of short seller firms

Yes I'm very familiar with them. 

The difference is that an activist short seller serves as his own catalyst. The successful ones have all created a platform for themselves with a large audience. Because of that, when they publish on a name, they can cause the stock to immediately drop. In contrast, if you are at a "normal" HF, you would short a business quietly and hope that your thesis plays out over time. 

Because activist short sellers can catalyze the stock with their reports, they usually buy puts and close out of the name shortly afterwards. Most of them don't stay in the name long term. The only difference I'd point out is the nature of the shorts themselves. Activist short sellers tend to focus on rooting out frauds because that is what has more impact. At a normal fund, most of your shorts will be more fundamentally-driven. 

I personally believe they activisr short selling is a highly successful and profitable business model. But it's not for everyone. I've spoken to a number of them and you have to be willing to put yourself out there. Many people view short sellers as market manipulators and the face of evil. So every activist short seller I have I have spoken to has faced personal litigation from their targets, death threats, etc. I have been told some really strange and scary stories. So it kind of depends on your personality.