There's no check valve. If there was, water would be trapped once the hose is disconnected and could possibly freeze. The flush piece just has a couple strategically placed holes that allow the water to spray in multiple directions. It doesn't take much to clog them off as stated by Avid. I guess it's possible that a little TP or other waste could be on the end if the tank was completely full and wasn't flushed, then dried.
Also as soon as the hose is disconnected the remaining water runs back out of the fitting. That's its normal operation.