Chris Webb takes us through the Diagnose Step button in Power Query:
As you might have guessed, it’s closely related to the Query Diagnostics functionality that was introduced back in October. Whereas the existing Query Diagnostics functionality allows you to see what happens inside the Power Query when a query is executed, this new feature does something similar but allows you to run a query up to a specific step. This is useful in scenarios where you want to reduce the diagnostics data you are collecting to a subset of the steps in the query without having to comment out a lot of M code.
It looks pretty useful.