What is a market mover in horse racing?
A market mover is a horse whose price is being cut by bookmakers faster than the rest of the field, usually because money has come in that the bookmaker considers informed. Most market-movers pages, including the well-known free ones, show this after the move has finished: a list of today's biggest price cuts, refreshed periodically through the day.
The data on backing a market mover after it has already moved is clear: steamers backed at the moved price lose money against Betfair SP. The profitable side of the same data is the price before the move completes, while some bookmakers are still showing the earlier price and others have already cut.
HorseWhale watches for that gap directly. An alert fires while a price is still being cut across the panel, naming which bookmakers have not caught up yet. Every alert is graded afterwards against SP and Betfair SP, in public, on the receipts page.