Chris Webb explains an ambiguity:
This query connects to the TripPin OData feed (which is public, so you’ll be able to try this yourself) and filters the People table so it only gets the rows where the FirstName column equals “Angel”. If you paste this query into the Advanced Editor in Power Query Online to create a dataflow, you’ll see that the filter shows the “Might Fold” step indicator:
Read on to see what that indicator looks like, as well as an explanation of how Power Query doesn’t always know whether or not something is foldable.