Updated: Oct 4
One of the goals I tried to achieve with my layout was to keep it as close to the standard QWERTY as it was defined in ThinkPad X-Series laptops (up to x220 model.)
The biggest deviation from that layout were the Shift keys which were moved under thumbs. Other modifiers: Left and Right Control, Alt and AltGr, Win, and the Menu buttons remained under the same fingers.
The latter is quite important. In ThinkPad laptops I pressed Controls with pinkies, Alt keys with the middle fingers, Win with the left ring finger, and Menu with the right ring finger. The same fingers are supposed to be used with ThumbsUp! keyboards:
Shift key is pressed/held with a thumb, it is just dropped on it:
Control key is pressed with pinky:
The Control+Shift combination is pressed with thumb and and pinkie together:
I learned that some people treated Win, Alt and Menu keys as an extension of the thumb cluster, and found it a bit awkward. Indeed those keys are are much easier to reach with the middle and ring fingers. I press them on the very edge of the caps. In my own boards I even rotated the key caps so the slanted edge looks upward, much easier to catch it with the finger tip:
Sometimes I curl the finger and hold the key with the finger joint. It may seem unusual, but it got its benefits: I just bend the finger, its tip slides over the key, and the wrist drops down. This way the wrist is not lifted nor it is bent and held upwards by itself (which seems to be one of the miniscule repeated motion leading to RSI). Instead it is pushed up by the finger moving under it.
Control+Alt is the trickiest combination and somewhat an exception from the finger assignment rule. To press this combination the easiest way is to move the palm sideways, the pinkie automagically moves to Control key, and the thumb moves over Shift to Alt:
It is a bit hard to see those fingers placement as the rest of fingers are relaxed and cover the view. So for this picture I lifted those unused fingers:
Thumb is used to hold Shift+"Cursor L" and Shift+"Cursor R" combinations (for instance when you type !@#$%^&*(), the numbers with Shift.) For it the thumb is dropped in between the keys, pressing them both:
That's all about the modifiers.