It isn't:
A jump drive causes a ship to vibrate in multiple dimensions until its modulations are in perfect synchronization with the subspace continuum. A vortex opens, creating an aperture from an infinitesimally small point in the cosmos, enabling the vessel to cross the subspace threshold.
Taken from the techroom description and in extent the fs-doc.
The vibration takes effect on the ship and not the space around it.
IMHO the vortex is how humans percieve the transition from normal-space dispersion to subspace dispersion of all your osciallations.
The reason why you can't use shields inside subspace IMHO doesn't have anything to do with polarisations (there was no whatsoever reference to such a nature of subspace).
It was also never mentioned that you need power to maintain jump phenomenon - you could have disabled ships in subspace and they don't fall/jump out.
IMHO the reason is far less the impossibility of using shields in subspace - My opinion is that it IS possible to use them.
The reason why no one does is the fatality of doing so.
Shields force jump anything thrown at you at a random vector. However Newton 3rd law still takes place - beside putting in the needed energy you also have to deal with the inverse force.
Since the rocket/laser/beam doesn't hit you, you can argue there is no such thing - but it hits your shield, and when it force warps the incoming threat it changes it's vector as if a force was applied in THAT dirrection (a term I have to use when speaking of using subspace).
The inverse force takes its toll on the bearer of the field - you. Therefore you will be somewhat warped moved in THAT dirrection too.
Imagine what happens if you did that in subspace: your vector will be suddenly altered.
Even though you may avoid the attack you can be lost in subspace forever. (Maybe that's what happened to the Nyarlhoteph).