The PM and team lead write a description of a task, and the whole team reads it together, thinks about it privately, and then votes on its complexity simultaneously using a unitless Fibonacci scale: 1,2,3,5,8,13,21... There's also a 0.5 used for the complexity of literally just fixing a typo.
Because nobody reveals their number until everyone is ready, there's little anchoring, adjustment or conformity bias which are terribly detrimental to estimations.
If the votes cluster tightly, the team settles on the convergent value. If there’s a large spread, the people at the extremes explain their thinking. That’s the real value of the exercise: the outliers surface hidden assumptions, unknowns, and risks. The junior dev might be seeing something the rest of the team missed. That's's great. The team revisits the task with that new information and votes again. The cycle repeats until there’s genuine agreement.
This process works because it forces independent judgment, exposes the model-gap between team members, and prevents anchoring. It’s the only estimation approach I’ve seen that reliably produces numbers the team can stand behind.
It's important that the scores be unitless estimates of complexity, not time. How complex is this task? not How long will this task take?
One team had a rule that if a task had complexity 21, it should be broken down into smaller tasks. And that 8 meant roughly implementing a REST API endpoint of complexity.
A PM can use these complexity estimations + historical team performance to estimate time. The team is happy because they are not responsible for the PM's bad time estimation, and the PM is happy because the numbers are more accurate.
A clear description with background appears in Mike Cohn’s original writeup on Planning Poker: https://www.mountaingoatsoftware.com/agile/planning-poker