It's the combination jk (or kj) that is set up to switch from insert to normal mode. The idea is that this combination rarely occurs in English text. The drawbacks are a slight delay in cursor movement when typing j or k as vim waits to see what comes next, and momentary confusion when you inevitably need to actually insert these sequences.

I don't think the speed hit you take when you actually have to type 'jk' should be a dealbreaker. It rarely comes up, and when it does, you can use e.g. '<C-v>j<C-v>k' or 'jjkak'.