During my previous co-training with Simon Bennette, we have come up with a new process to re-define the true meaning of estimation, and we call it "Estimation Mapping".
Exercise: Using Impact Map method to find the goal "How to make estimation beneficial for everyone".
Experiment: The participants were slightly confused in the beginning (I suppose because we all think estimation is mostly for management to establish deadlines for the team). However, they proved themselves such an open-minded team and they completed the exercises with all the "deliveries" that were all about "communication", like "establish the same understanding for PO and Team", "see if the feature is worth developing" etc...
Conclusion: Simon and I are still working on refining the exercise; but we, Simon, me and the class, all reach to agreement that "Estimation is not about deadline, estimation is about communication."
[This post assumed that you understand Impact Map, http://www.impactmapping.org/]