If I am in a combat scenario and someone told me to jump (because something is trying to take my feet out from beneath me / the ground is breaking apart beneath me), then it would be the height of stupidity to stand around dumbly and ask 'how high?' instead of just reacting
The whole point is that the command "Jump!" comes out of nowhere.
They are not in a situation where jumping is required, expected, or normal. Imagine them just walking down some supermarket's aisle shopping for stuff, among other shoppers. For no reason at all the boss commands "jump." That's such a weird thing to command at that moment, it's absurd... so obviously the subordinate asks "how high?"
Do you see how it's a betrayal of expectation that he asks "how high?" and not "why?"
That's part of why this adage is successful, because it sets up that the subordinate is asking a question, and the question that the reader/listener would expect to hear asked of a preposterous, stupid, unwarranted command is "why on earth would you want me to do that?" or even "why should I do that?"
But the reader/listener is surprised by the ACTUAL question that the subordinate asks, which ends up leaving in their minds a stronger emotional impression than the boring "Jump!" // *jumps* would.
It's a powerful, memorable, example of absolute loyalty.
I am trying to figure out how asking 'how high?' when being told to jump, is a signal of trust instead of stupidity
Okay, practically this story: "We were walking down the aisle, the boss said 'Jump!' and I instantly jumped" might technically be a better demonstration of obedience, but it doesn't subvert any kinds of expectations like the original does, doesn't play on common patterns (call-and-response), it doesn't enjoy any of the advantages of the actual adage that make it so memorable.
I would also say that perhaps the metaphor is not just used to establish the subordinate as a mindless robotic servant, but as an unwaveringly loyal human striving to acquiesce to every requirement of his liege. So asking for specification "how high?" could imply that the subordinate is aware of their subservient status, happy about it, and is even proactive to reinforce it. It's a stronger kind of loyalty than the "logical" instant-acquiescence is ("Jump!" // I must be in danger! *jumps*).