I have a theory I keep reciting, I hope it will give some ideas this time as well:
You can enter subspace where it is aligned enough to form a subspace-field.
It is actually ordinary space created from the extra 6 dimensions that all elementary particles exist parallely with our known 4 dimensions (this is actually the latest in loop-physics, I didn't just make up the part about the 10 dimensions).
The problem with those is that most of the time they're folded up, so they don't interfere - or do it to an impossibly small degree, does 10E-31 ring any bells? - with ordinary space.
However it is possible, that although they don't connect to our 4 dimensions seemlessly they can form a space on their onw - and it doesn't have to have the same proportions as normal space - so you can travel faster in subspace.
Subspace are only used to align subspace with normal space and make an entry possible.
When the ship enters it actually "turns" into subspace. What? What the heck is that supposed to mean?
Well lets take 1D - linear - example. If an object existed in a 1D universe, where 1 more dimensions were folded up, you could model the universe as line on a plane where each point of the line has another dot assigned to it.
The random array of dots is actually subspace - they can form another line - another "space" - but you could go onto that space only in places where they interlope.
When "turning" the ship exists in multiple dimensions at once - its inside the dot where the spaces interlope, than its proportions get into another dimension, in a manner like rotating a paper - its dimensions among the 3 vectors of space change - if the object is in 10 dimensions it could rotate in an infinite more ways.
Well that's my principle idea of subspace.
It still does not answer why do nodes and jumps behave as they do. Here comes another part: gravity.
The techroom mentions that intrasystem jumps require less energy, but it needs a close, powerful gravitational field.
Gravity can be handled as if it bends space - subspace as well!
So where gravity is stronger it is easier to enter subspace, and subspace itself is more coherent allowing faster travel.
So each object with significant mass is generating a subspace bubble arroung itself - for anything smaller than a planet, the effect is insignificatn, but this explains why the Psamtik(?) missjumped when the Shatanes came in - the Shat was so huge it distorted subspace just a bit.
Nodes are the places in space where the gravitational pull between two stars is the greatest.
However tunnels existing between two stars' subspace bubble is a special place - the lack of lateral pull makes space inside the tunnel circular - if you kept going normal to the tunnel, you'd reach your starting point.
This has an unexpecteds effect: Electro-magnetic Waves (anything from infra-light to gamma-rays) and charged particles along with matter can get trapped in the middle of the tunnel.
The nice thing is that this energy has mass as well - that's actually framing the tunnel.
During the Big-Bang huge ammounts of energies flooded subspace - a decent percentage of this was trapped in tunnels, when the stars started drifting aside.
Each star also dumps some of its energy into the tunnel, so whenever the stars got further apart the tunnel was still supported by the energy frame.
This is the reason why shields cannot be used in subspace, they would be blown away by these energies in a matter of seconds.
They - most of the time - don't harm passing ships though, since this energy is on a stabilized *sometimes oscillating* "orbit" inside the tunnel, that can be avoided, shields on the other hand form strong electro-magnetic fields that channel these forces.
It would be a nasty thing to sabotate a ship by switching its shield on inside subspace - the fighter would feel the power accumulated during eons. Fried Terran anyone?
If the PVD hope was stuck in the middle of a subspace tunnel it would start to drift toward one of the stars - or if its in exact position the stars pull could equal each other.
The ship would also keep its momentum in subspace.
It would, drop out though.
It actually takes energy to get out of subspace as well.