Nick, we agree practically, but not entirely
Firstly
Newton's Second law states.
The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction.
In reality we feel intuitively that the force must come first, however they are instantaneous and inseparable.
In a balloon that becomes untied, pressure is used to accelerate the air in a specific direction so producing a reaction force. What came first momentum change or force?
In a jet engine fuel is burnt and gas increases in velocity so producing a reaction force. What came first the change in momentum or the force?
On a helicopter rotor disc, as the blades flap the 2nd moment of area reduces (CG moves inwards) reducing the angular momentum. A tangential lead/lag force is produced. What came first the change in angular momentum, or the force?
The answer to the above is that they both occur instantaneously.
Thought No.1
If a body is moving with constant velocity, and the mass of the body is changed instantaneously (without momentum change) then what happens to the velocity? Newton says that it will tend to increase in order to preserve momentum. If it doesn't, then a force must be present to explain the change in momentum.
Thought No.2
How does a helium balloon generate lift. Where does the change of momentum come from? No mass is "let go of" in fact within the "bag" of the helium ballon no energy changes form. However gravity is no respector of boundaries, it re-arranges the air around the outside of the bag, this in turn generates the necessary newtonian conditions. The sums don't work until you include the atmosphere and the mass of the planet.
I hypothetically put a rocket in a bag last night, I managed to get to the Sun. Then again I was in the bag with it, and the bag was 150,000,000km across
You need to bound the system correctly. If Jiff's machine has a torque generation system that works by producing the reactive torque at a totally unconnected place in the universe, then his machine would allow us to move within that universe without Newton getting upset. This is the equivalent of the boy with the three mile arm.
Keep an open mind, remember,
Once the sun revolved around the Earth which was flat.
Manned heavier than air flight was impossible due to steam power not having sufficient power to weight.
Jiff's machine is possible, it just isn't practical at the moment.
He needs to find a bigger bag.