Clenching the teeth does nothing but strain and waste time. I believe this can be a form of mistranslation, as many times the energy of the head can be felt in the pressure of the teeth as well.
You simply focus of the feeling and sound, not on the actual motion of the tongue. As you place your focus on hissing and the inhale, or exhale you have succeeded. Don't strain your tongue, the tongue is delicate and precious.
Also you might confuse with Sithali? With the hissing breath there's no curling, but slightly moving the tongue up, close to the upper palate, just so you produce a hissing sound.
Thanks for clearing that up.
I think I am confused as when I place my tongue slightly up and inhale the air passes on each side of my tongue through each side of my cheeks but the instructions say to keep a small space and does not mention what I described. Furthermore, on reversed Sitkari I can place the tongue so lightly that air passes between a small gap between the tip of my tongue and the roof of my mouth but I can't do it the same way with the other variation.
I know this sounds confusing and over analyzed so I apologies in advance.