The word is only correctly used if the man feels emasculated as a result - it does not cover all situations where a woman has authority over a man. A female boss who just threatens to fire a guy would not have him in a testicle lockbox if he feels machismo because of it.
In the late 90's, perhaps the genesis of the term, it was used to describe the arrangement Hillary Clinton had come to with Bill Clinton after the Monica Lewinsky scandal erupted.
A: Naw, man, my old lady, she don't want me being out late.
Q: She's totally got you in a testicle lockbox.