Sorry, it's not you. I'm clearly struggling to put the concept into trigger-agnostic terms so I'll just give you a concrete example from personal experience instead.
I wanted Mouse Button 3 to act as CTRL+ALT+CMD+SHIFT, a sort of custom hyperkey if you will. I wanted the ability to trigger actions merely from a Left Click or a Right Click which would each require all of those same modifiers.
So I first created the following triggers and actions:
MB3 (on down) = CTRL(down) + ALT(down) + CMD(down) + SHIFT(down)
MB3 (on up) = CTRL(up) + ALT(up) + CMD(up) + SHIFT(up)
Left Click (Requiring CTRL+ALT+CMD+SHIFT) = Move Left a Space
Right Click (Requiring CTRL+ALT+CMD+SHIFT) = Move Right a Space
This got as far as triggering the Left Click or Right Click actions, but then upon releasing the button combination I was stuck with CTRL, ALT, CMD and SHIFT all in the down position. Obviously, pretty big problem...
The solution that actually worked was a modification of the above to the following:
MB3 (on down) = CTRL(down) + ALT(down) + CMD(down) + SHIFT(down)
MB3 (on up) = CTRL(up) + ALT(up) + CMD(up) + SHIFT(up)
Left Click (Requiring CTRL+ALT+CMD+SHIFT) = Move Left a Space + CTRL(up) + ALT(up) + CMD(up) + SHIFT(up)
Right Click (Requiring CTRL+ALT+CMD+SHIFT) = Move Right a Space + CTRL(up) + ALT(up) + CMD(up) + SHIFT(up)
That said, while the above did work, I ended up scrapping it in favor of a conditional logic approach. I did a little write up on the solution I settled on which you can check out here: Mouse Chords: How to Use a Mouse Button Click as a Pseudo-Modifier for Complex Mouse Inputs WITHOUT Karabiner, SteerMouse, Maestro, Etc