They are like the auto-guard/auto-tech gems except they require even less meter; some people abuse it unfairly so it makes it hard to play em online. I'm not sure if the gems are on PSN yet, but I hear its pretty annoying on XBL.
Edit: Ok it is on PSN, just took a look at the assist gems. Lvl 1 Ultimate Defense Gem: Enables auto-block and auto-throw escape, and no chip damage; costs one block.
I'll be honest, I haven't played too many people with it, but the few I have are pretty annoying to fight lol.