A
humbler is a BDSM
physical restraint device used to restrict the movement of a
submissive male participant in a BDSM scene.
The humbler consists of a testicle cuff device that clamps around the base of the scrotum, mounted in the centre of a bar that passes behind the thighs at the base of the buttocks. This forces the wearer to keep his legs folded forward, as any attempt to straighten the legs even slightly pulls directly on the scrotum, causing from considerable discomfort to extreme pain.
That is what WICKI says.
Many men like this activity as part of cock and testicle.
Many find it enjoyable.
To some, it may not be their cup of tea.