According to Newton's Laws:
Force = Mass x Acceleration
So if you have a body that is moving it has a corresponding energy, or force, according to the equation above and of course we're all aware of the conservation of energy:
Energy can neither be created nor destroyed but can be changed from one form to another.
Next step, assume we have two objects:
Object A: The Whacker (Moving)
Object B: The Whackee (Stationary)
If we whack one object A off object B we get a transferal of force from A to B. However, object B exerts it's own force back on the object A, this is the force required to accelerate object B from being stationary and into motion which is proportional to the force at which object A hits it.
Hence to every action there is an equal and opposite reaction.
Veracity of sciency stuff is not guaranteed.
or is it that the thruster is moving away from the jet blast / stream that its sort of pushing out / against
Pretty much: The exhaust from the ignited fuel is pushed away from the rocket which in turn exerts a force on the rocket in the opposite direction hence the rocket moves forward (or up)