Ending Explanation (Spoilers)
I didn't catch this until I rewatched the end scene a couple of times (I have a bootleg copy :-/). The director completely rushed it (rather poorly executed) through so some of these revelations occur within a couple seconds and are difficult to string together:
1. Fiona confesses that she was the girl who caused the accident
2. As Isaac is trying to leave, Fiona tells him that his paralysis is hysterical, as in it's psychologically manifested, not physically caused.
3. She asks, "What makes a person want to be paralyzed, who isn't?"
4. He has a flashback of the accident. Prior to this (as he mentions in an earlier flashback), his memory of the events leading up to the accident end at where his mom kisses his dad and throws him a hard candy, and he doesn't remember anything that happens after that. This time he finally remembers:
In the (slow-mo) seconds leading up to the accident, he had been playing with the lock on his side of the door. IMMEDIATELY after he pulls up the lock, Fiona's car plows into theirs, killing his parents. There's a brief shot of him as a blood-splattered boy waking up on the ground.
It's meant to imply that even though it's obviously only a coincidence that Fiona killed his parents right after he pulled the lock, as a little boy he internalized it as somehow he had caused the accident. Paralysis was a manifestation of his subconscious guilt.
The adult Isaac isn't conscious of this, obviously. Earlier though, his subconscious motivations are hinted at when Isaac is throwing "Ginger Jake" at Fiona and telling her that "You don't deserve to be paralyzed!" whereas she retorts, "And YOU do??" Fiona doesn't know about what happened during the crash, but the fact that Isaac was already equating paralysis with something deserved gives a clue.
It's essentially the same reason that Fiona consciously WANTS to be paralyzed by Isaac--guilt over the accident, tit for tat, penance.
5. Isaac, angry and emotional either because of her prodding, because of his sudden revelation, or because he's remembered for the first time the exact moment she drove into him, throws aside the coffee table and lunges at Fiona.
6. Fiona easily throws him out of his chair onto the floor, and walks over and sits in his chair looking at him. Thematically, this shot represents how the blame Isaac felt and internalized has now been properly transfered to Fiona.
7. She throws the shoes at him. But his legs start to move independently because now that he remembers the accident, he's conscious that he had subconsciously blamed himself and knows as an adult it's completely unreasonable. HE obviously hadn't caused the accident, Fiona had.
I hope this helps. I thought the acting was good but the plot kind of hokey, and that this climax was poorly executed. As a psych major though, I appreciated the psychological depth and accuracy the director/writer put into it.