Comment by sgarland

Mr Chat more or less covers it correctly.

If my memory serves correctly, in the book there's a bit more emphasis and nuance on the point that, for buy-in to exist in the first place, all team members who have a strong opinion prior to a decision, need to feel that they've been given an opportunity to give their opinion, their opinion has been heard, and its merits and demerits weighed earnestly against all other options on the table, before a decision has been made by the team (or in the case of a 'stalemate', by the ultimate decision-maker), and that there is a clear rationale for going with the final decision and rejecting all others. The rationale may well acknowledge the risk from not following other opinions, but cite other operational reasons for the decision.

A healthy team then should proceed as if this was a team decision that they all commit to. But consensus isn't required, only buy-in. In a healthy team, all members should trust each other (in that, they are all working towards a common purpose), and not fear "creative conflict". This enables them to share their opinions in the first place, and the buy-in discussion helps everyone in the team to feel accountable for the joint decision, as well as hold each other accountable in a healthy way. If this doesn't happen, the usual outcome is that decisions are made, but people feel "I didn't agree with this decision and nobody asked me, so I won't really engage with it".

I really do recommend the book. It's a very light read, presented as a story (with a final chapter examining the concepts a bit more theoretically), with excellent insights. You can probably find it online for free if you must, but it's a good book to have on any bookshelf.