Sometimes used as an initiation routine by gangs they set a new member a kill clock where they have to kill a set number of members of a rival gang in a set time.
Sometimes used as a form of blackmail that if a gang member doesn’t complete a kill clock someone close to them may be harmed/killed.
Sometimes organised by inmates wanting to get to someone on the outside using their gang-contacts.
"You have 24 hours to kill as many rival gang-members or someone close to you will pay... The kill clock is ticking"