Quote from syndutrill >>

> Okey, so it's immoveable, but in which inertia system? 

exactly. why would you assume it's only relative to the planet's rotation and not it's orbit around the star, or the star's orbit around the galactic disc, or ... 

for example, earth rotates at about 600 miles per hour, orbits at around 6000 miles per hour, and moves through the galaxy at around 60000 mph.

imo, it needs to fixed to the inertial frame of the activator. so if you're riding on a dragon when it's activated, vs. standing in the same room as the dragon, it's going to give different results.

IMO, the rod, upon pushing the button, instantly fixates itself in place in 3 Dimensional space relative to the nearest, largest source of gravity. Therefor, the planet you're standing on would be that, and would lock itself to the planet in movement through the galaxy. There is no inertia involved, except that of the planet, in which you're already part of anyway.

If you were riding a dragon flying along inside the gravity-well of the planet, depending on where you pushed the button relative to yourself (above your head, in front of your stomach, etc), either your hand would instantly be destroyed or you and/or the dragon would practically instantly die from being sliced by the rod, because it instantly fixated itself in position relative to the planet and you and the dragon are moving very fast relative to the planet. Your body's inertia then, relative to the planet, would apply that force onto the rod, which is not going to move. It's like a seatbelt in a car if you have a head-on crash. You will be bruised in the shape of the seat belt. But if you're going 400 mph in that car crash, and the seat belt instantly stops you, your head very well may just rip itself from your spine. Yay physics!

Length of rod?

Imivabel shotgun glyph fireball to launch darts 24 darts in one turn

By itself: 
 - lock doors
 - pin enemies 
 - stop machines with gears etc. 
 - win bar bets
 - stop a wheel from making a full rotation
 - anchor small vehicles (think parking break if there's a space to slot into the frame of the vehicle)
 - instant fulcrum
 - pranks

With block and tackle + enough rope
 - instant manual crane (just use the rope as harness for the creature using the crane), add counterweights to easily transport people and objects.
 - move objects weighing less than 8,000 lbs horizontally given enough creatures that can pull/drag. 

Tinkering your own device from 2 or more Immovable rods:
 - Think stilts with the ability to push the immovable rod's a button by pushing a button. Have two of these and you can walk the horizontal and vertical plain slowly out of combat.

Big damage: (think boulders on a cliff kinda setup)
 - set up a rig that can hold objects of significant weight (enough to cause damage when dropped) and prop it up with immovable rod. Activate the trap using your own smart solution or go the easy route and sacrifice a familiar to do so.