What I've learned with Judah, and what I am also seeing in my students is that they are slow to process commands. To the common person this appears to be defiance, when actually it's just the kid with DS processing what they were told/asked.
This requires a lot of patience on my part. I'm used to my girls listening and doing what they're told/asked the first time. With Judah and my students I often get frustrated and on occasion lose my cool with them for taking so long. I almost have to unlearn and re-teach myself to be patient, say the command about three times and then expect the appropriate response.