Pull system, the most famous implementation is the Kanban
system.
Kanban is not necessarily the simplest pull system. The most
straightforward way to establish a WIP cap.
Given production line, establish a limit on the WIP in the line
and simply do not allow releases into the line whenever the
WIP is at or above the limit.
We call the protocol under which a new job is introduced to
the line each time a job departs CONWIP (constant work in
process).