Clever bit of molecular design! It’s a DNA hook that folds and binds to itself. It also has a site for binding to a partner. If you pull gently on the partner it just unzips from the hook and easily detaches. But, if you pull harder the hook unzips instead. Then the partner has to shear instead of unzipping. That’s much harder.

So that’s the “catch bond”. It comes undone if you pull gently, but the bond gets much tougher if you pull harder. Its a bit like those finger trap toys.

Also, many cells use this kind of catch bond for detecting and responding to biomechanical forces.