Tuning plays a huge role on if you can do the spit-note or not. The call has to be tuned with the ability to do it. Since you can do it on the rnt, I'd say a tune-up might be needed for the SMH.
I, personally don't stick my call when doing it....although it's close. I start with my tongue between my lips....like I'm going to spit a hair off the tip. After the initial burst, I then drop down to the "Awww" part of the note,dropping my jaw as I do.
Check your PM's, I sent you some info on how some of the pro's do it. They can probably explain it better that I can
Causing permenant flight delays
Callin' 'em in and knockin' 'em down