[SteveH]

or with a paperclip, a bit of wire, or a thin piece of metal

https://www.youtube.com/watch?v=-jo5A6yN_qc

[Paal Gulli]

"or with a paperclip, a bit of wire, or a thin piece of metal"

A key is easier (unless you're Harry Houdini).

[sgt_lau]

My go-to handcuff key was a hairpin clipped to the rear pocket of my pants. Attach it as close to the pocket seam as possible. If a pat-frisk is done it can’t be felt and if cuffed from behind my hand is in position already. If the cuffs are double locked I’m beat though.