I've read the whole of A Fire Upon the Deep and I still can't figure out what an ultradrive spine is? Is it an engine or something else?
In A Fire Upon the Deep Vinge has a tendancy to throw ideas around with very little discussion. Personally I love this style of writing because it makes for wonderful scenery without the tedious expositions that hard Sci-Fi authors are prone to. However it does mean there is simply no answer to your question. It's clear from the book that the spines are just a component of the ultradrive - the ultradrive still works with some spines missing or damaged. My interpretation is that the spines couple the ultradrive to whatever facet of the universe the ultradrive interacts with.
The Ultradrive technology
Only works outside the 'slow zone'. The galaxy in Verne's universe consists of 'zones of thought', the level of technology available depends on which zone you are in.
The closer you are to the galactic core, only rudimentary tech works and FTL is not possible... hence, the 'slow zone'.
The further out from the core, more and more advanced tech becomes operable. The spines are merely an element of the jump propulsion system of the Ultradrive.
A Fire Upon the Deep: The novel posits that space around the Milky Way is divided into concentric layers called Zones, each being constrained by different laws of physics and each allowing for different degrees of biological and technological advancement.
The innermost, the "Unthinking Depths", surrounds the galactic core and is incapable of supporting advanced life forms at all. The next layer, the "Slow Zone", is roughly equivalent to the real world in behavior and potential.
Further out, the zone named the "Beyond" can support futuristic technologies such as AI and FTL travel. The outermost zone, the "Transcend", contains most of the galactic halo and is populated by incomprehensibly vast and powerful posthuman entities.
It's certainly propulsion, but I'm not sure that "engine" is entirely accurate. In the Beyond and especially the Transcend, what would be normal physics goes haywire, and things that would be impossible here (the Slow Zone) are quite typical. As far as I understand the book, you don't even need reaction mass to move.
Vinge is quite hard on scifi and as John Rennie points, Vinge writes throwing big ideas and leaving fine tunning to the readers.
I got to the point where the drive spines where conducting the interaction between the ship and the medium/high Zones.
Remember there is a physics effect recalling the use of spin-like extensions - this is usually known as antenna - to concentrate electrons at its sharpest point, where space tends to zero (the last -metal- atom actually composing the antenna body) and charge or whatever desired physical effect takes place at its maximum expression (this is roughly valid, antenna structure may change this).
Long story short, yes, the ultradrive spines are kinda exhaust, or power transmission at least in a "normal" universe, which leads us to the undefined physics layered universe drawn by Vinge. Thus you can imagine the spines cover the entire ship isolating its local field from the outer field and producing motion through physical space at insane speed.