An uncanny ability

[Whisper]

I think its more body language that theyre reading, than words they understand. Stand in the middle of the stairs, face away from the dog, fold your arms, look upward, and say in a monotone "go upstairs", and see if the reaction is the same

Dogs are MASTERS, Jedi Knights, at reading body language and inflection

Words are mostly babble when not connected to the all-important posture, intonation, gestures, etc

Someone once told me to do the same thing with Millie. Face away from her, make my body posture look like I'm ignoring her, etc. She still listens. When I fold my arms. When I look up. When I look down. When I speak in a "baby voice." When I speak in a normal, offhand voice. When I act like I'm talking to another person. Even when I'm an in a room with the door closed! If she understands the words, she responds.
Dogs are absolutely body language ninjas. Millie knows when I'm sad, irritated, bored, anxious, etc. I'm sure she also picks up cues that I'm not even aware I'm giving. But dogs also understand more than we give them credit for.