Ways not to lock your bike

It was a few years ago now but someone once locked their Brompton through my bike outside my office in Canary Wharf. I proudly just folded the thing up and passed it through my frame.

This needs at least a picture drawn because my simple brain fails to grasp how you did this.....like those bloody wood and string puzzles with a ring you have to release....unless you used magnets...then I get it....