The 1/3, 2/3 rule is a rule of thumb for planning. It is intended to make sure a leader gives her team enough time to complete their work.
It is a Video Term from Phase 3 (Starting the Journey) of our Continuous Improvement Development System.
People on continuous improvement project teams have constant pressure on them to leave the team to go take care of things in their day to day job. This rule of thumb can help them decide when to leave.