Well, you don’t, however your probability of getting a slow line (in the example below) is 66%, which is why you always seem to come out worse off. Here’s a great video to explain, via Lifehacker. (Young ‘uns, you might want to read this first.)

(Every time I stand waiting at the Tesco self-serve checkouts, I wish they’d formalise the combined queue. I actually map out the rope barriers in my head…)