Welcome to knowing the awesomeness of Garou, M_C.
Just defends are pretty simple, it's just blocking at just the right time, not that different from parries in Streetfighter III. Just Defends have a few advantages, they give you a little health back and allow you to cancel into a lot of moves. The cancelling aspect isn't really hard either, just input the move during the just defend animation.
It's all down to timing really, like a lot of things in fighting games are.