thanks for asking.

I'll just answer with a cause and effect overview for the two:

toggling a modifier: 
first button press => modifier key is held 
first button release => nothing 
second button press => modifier key is released 
second button release => nothing

holding a modifier: 
first button press => modifier key is held 
first button release => modifier key is released 
second button press => modifier key is held 
second button release => modifier key is released

Hope that explains it.

Best regards, 
Felix