rendering part of a model first

I was wondering, I've seen some games where only part of a model comes into view. For example, like a zombies hand going through a wall. You don't see the rest of the zombie. I was wondering if someone could have a model partly rendered like the top or bottom. I could make a part of the model, like just the arm.

You can split the model or hide it behind other geometry. What's the problem?

I was just wondering if part of the model could be rendered first.

Thanks