If a mortal were to say “sit down”, he is giving you an instruction, or should the mortal say “please sit down” he is being polite.
“Sit-down” is different: the hyphen (guion) links the words to form a singular adjective.
• Sometimes workers go on a “sit-down strike” (una huelga de brazos caídos), meaning that they protest by stopping work, but stay at their desks.
• There can be a sit-down protest, a similar concept.
Also, mortals are sometimes tired and might say “I need a sit-down” (…sentarme a descansar un rato) meaning that the person needs a rest.