Thank you! So it's actually something we're still figuring out. The first few characters there is a pre-processor to look for some commands or vertical search (not really fully implemented yet). For question-answering we're looking at a new approach but it's not good enough to deploy yet. If it looks like a web search then after the first few characters, it will generally go to the Bing auto-suggest API currently through an anonymized router. It's a handy convenience but it could be much better, so lots of work to do on this.
