Over the years there has been much speculation and confusion with appropriate use of FLCH. When I have come across such confusion (many times in 12,000 x 777) I have explained that it is an IAS lock, much the same as the IAS button on the MD 80 series. There are a few other things that attach to it but essentially it will try to maintain selected speed.
Others have explained that there are traps for the unwary, however if you know what is going on it is a safe and expedient way to manage your flight profile. Frankly it was my preferred (non STAR) mode for descent prior to G/S capture (from above), particularly when operating in a speed controlled environment.
Maui.