Have just obtained a new piano reference book, which comes fairly highly recommended. I am focusing on technique at the moment, doing C major without scrunching the thumb. I eagerly sought out the relevant passage in this new tome.
Anyway, "The Piano Handbook", by Carl Humphries, first edition 2002, p51:
"After we play the first three notes with thumb and first two fingers, we pass the thumb under after the 3rd finger and place it on the fourth note."
and a bit later
"When passing over or under like this, be careful not to let your elbows stick out: make your wrist and fingers/thumb do the work."
Obviously thumb-scrunching is not as frowned upon in some circles as others. What's the deal with the elbows? To make room for the thumb to hit that fourth note without going under the palm, while keeping finger 3 on the prior note for the legato scale, logically the hand has to "yaw". To do that, it has to either bend at the wrist or out goes the elbow a bit to rotate the hand. Also some techniques on youtube appear to roll the hand, meaning the forearm is doing the work, not the fingers.