Here's a variation on the same theme:
This rod works in exactly the same way, by compressing the back of the neck.
It mounts in a different way, though:
This rod has a threaded "slug" at each end, one with right hand threads and one with left hand threads:
Turning the nut to the right draws the slugs together, compressing the neck and bending it backward, decreasing neck relief.
Turning the nut to the left forces the slugs apart, effectively stretching the back side of the neck, causing the neck to bend forward, increasing neck relief.
This two-way adjustable truss rod is very effective, but slightly more fragile than the standard rod.
Bending Rods
Look at this one
This is really the same mechanical system as the standard compression rod. The rod is embedded in an aluminum channel, with a nut at one end. I've taken the nut and washer off so you can see the channel better.
A heavy anchor forms the other end of the rod:
When the nut is tightened, the rod pulls hard against the channel. The open side of the channel compresses easily, while the opposite side resists the pull. As a result, the whole business bends toward the open side of the channel. This style rod is installed with the open side of channel facing the back of the neck, so that under tension the neck bends backward along with the rod.
The operative difference here is that the rod system itself actually bends, taking the neck with it.
Here's the channel system in its natural habitat:
This is the end of Martin neck removed from the body, shown upside down. The nut is accessible from the inside of guitar body, through the soundhole.
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