I was watching Chuck's review of Star Trek VI to pass some time and of course, it starts with Praxis going boom. We've hashed out that the explosion seems to grow stronger the further away it travels from Praxis, meaning that it can't be an explosion as we know it. I've also been playing Star Trek Online, and the supernova that blew up Romulus was supposed to be triggered from the Hobus system and used subspace to travel to Romulus and destroy it. Now, according to Star Trek Online, the detonation was caused in secret by the Remens using protomatter. So... what the hell?
We have two instances of explosions that seem to travel through subspace and cause interactions with normal matter while at the same time gaining additional energy as it expands. According to Memory Alpha, subspace communications do not suffer from the radius squared degradation common of real space communications, so in some way it acts in ways that aren't linear as we understand vectors travelling through space. There are certain particles, such as tetryon particles, that can only exist naturally in subspace, so there are vast differences between what we can observe in real space and what we observe happening in subspace.
One thing I noticed while doing a bit of research for this article is Starfleet's reliance on subspace amplifiers and relays. With the statement that subspace communications do not suffer from radius squared degradation, it would seem that any communication sent from anywhere in the universe would be heard. So why would there need to be relays? I put forward that subspace doesn't operate entirely in three dimensions but rather in a more limited 2.5 dimensions. We typically see starships maneuvering in two dimensions, with the third only used to allow ships to maneuver up and around. If that's the case, it takes more energy to shift in the third dimension than it does in two, focusing energy into a band of two dimensional expansion.
That would explain the ring from Praxis, but doesn't explain how the ring gained energy as it travelled, since it would still be subject to the radius squared law, or how communications still need relays. Well, Geordi once referred to subspace as "...a huge honeycomb with an infinite number of cells". So what if that's true? A two dimensional field made up of cells, with a barrier between each cell.
This explanation lends nicely to the idea regarding Praxis, in that we saw a huge explosion that rippled out, but it didn't damage anyone beyond the sector they were in that we're aware of. So what if the Praxis explosion expanded through subspace, creating a ripple through the particles already present in subspace and then dissipating when it hit the cell's border? If that's the case, then by using amplifiers, they can bridge the sector's barriers and allow real time communications. And since starships can breach the subspace barriers, over time they would begin to degrade, as explained in TNG.
So... that explains everything except the supernova. Wouldn't the energy in the supernova simply dissipate as it hits the sector barriers? Well yes, except that by using protomatter, which has been observed to reignite stars, the star's energy was greatly enhanced and forced into subspace. That much energy is far, far beyond the energy commonly observed in subspace travel, communications, or even during the Praxis explosion. That much energy could reasonably breach the sector's barriers and continue onward. It would severely damage subspace as well, making any investigation into the explosion even harder (though this gets written off when your character conveniently shows up to solve the flipping obvious).