Ahh- well as Heidi says I work with the ones that people have given up on pretty much. Some of them are so aggressive out of the gate because of the treatment they got while they were loose in the world that the only way they feel they can protect themselves is through aggression. Being launched on by a ticked off cat with claws splayed is not my idea of a good time.
The cardboard keeps me safe, and them safe as well because their claws can't grab the slippery surface until they figure out how.
Sadly, the worse case of aggression that I have ever dealt with, was not a feral cat, but a domestic. Just a young one, whose owner contacted me and begged me to take him from her. I told her I would and would return him back to her once his behavior mellowed. She actually drove him here, she lived in Idaho. She brought with her, a computerized litter pan, two cloth cat carriers, a whole trunkful of toys- I kid you not. I opened the carrier upstairs in the cat room, he spit and dove straight for me! I ducked my head and turned a bit (I was kneeling) and he overcompensated and sailed over my head. By the time I stood up, he had launched himself at me again, so I quickly got out of the room. He was puffed up like a little puffball and spitting and actually lunged for me before I got to the door. I was amazed this was a cat this woman had owned from a kitten!
I came downstairs, fixed a pot of tea and started talking to Gladys. She was a very articulate woman, and through the conversation she started telling me some of the horrendous things she had done to this cat!
I won't go into it here, but the acts were nothing short of cruelty and she would instigate them for reasons such as "he made the litterbox smell to bad...he wouldn't quit crying....It got to the point that this cat was so on guard with her that if she came anywhere near him, he would lunge at her and try to hurt her. Who could blame him? After all, she terrified the living daylights right out of him! She told of one instance where he kept her pinned into the corner of a room for over an hour pacing back and forth and growling! I wanted to throw her ass out of the house, but I sat there just gathering information and trying to stop from telling her she needed to go play in heavy traffic! Finally before she left, she said she didn't want him back, he was "my problem!"
Well Terror which i renamed immediately, took about 6 months of constant reassurance and TLC before he even calmed down. Then it was about 3 months more before he accepted anyone else in the room but me. Eventually he was rehomed to a young man who loves this cat very much and calls him Clyde. They both are doing well.
Cats are shaped by their genetic traits and their life experiences. They don't have to be ferals to be threatening, but if you treat them the right way, the rewards are immeasurable.