He was on the sixth stair when there was a loud, wailing, klaxon-like sound and the steps melted together to make a long, smooth stone slide like a helter-skelter. There was a brief moment when Ron tried to keep running, arms working madly like windmills, then he toppled over backwards and shot down the newly created slide, coming to rest on his back at Harry's feet. (OP17, Bloomsbury edition)
A helter-skelter is an amusement park ride, often in the form of a lighthouse, which consists of a spiral slide running around the outside of a whimsical structure. Riders sit on burlap sacks or other simple things like that and slide down.
late 16th century (as an adverb): a rhyming jingle of unknown origin, perhaps symbolic of running feet or from Middle English skelte ‘hasten.’
The Scholastic version of this passage ends with "stone slide" and doesn't include mention of a helter-skelter.