I think the tube shank is how you adjust the needle spring tension, same as on other brush brands. I don't think supposed to be screwed all the way in tight unless you want the trigger pull to be as heavy as possible.
I usually remove the handle if I want to unscrew the chuck. With the handle off, you can just pinch the tube shank with the thumb and index finger of the hand holding the body while you unscrew the chuck with the other hand.
Of course, I also chop the plastic bulbs off the back of my SOTAR needles. If one doesn't do this, the handle can't be removed without removing the needle first. I highly recommend chopping the bulb off the needle for a variety of reasons. IMO it's a bad design element that buys 1 or 2 negligible/unlikely to be used advantages with several severe/highly likely to be encountered disadvantages.
If your needles are bulb-intact, you can just pinch the bub with the hand holding the brush while turning the chuck with the other. The needle tube is bound to the needle as long as the chuck is tight enough to grip the needle, and it's the tube that's transferring rotation from the chuck to the shank, so if you immobilize the needle itself, that will immobilize the whole part chain. If the tube or shank rotates independent of the needle while rotating the chuck, that means the chuck is already loose enough that you can remove the needle.
Either way, it's simpler than sticking something in the trigger slot, and doesn't require you to keep an extra tool at hand.
If you chop the bulb off the needle, this also enables you to remove the needle by removing the head and pushing the needle through the front instead of pulling it out the back. I prefer to do it this way as it removes the risk of dragging paint gunk into the back mechanisms of the brush (think about it: if the packing isn't wiping the needle enough to make wiping it after removing redundant, that means you've also just wiped paint all through the mechanism by pulling the needle out through the back).