Does what you'd expect and securely holds 2 rockboard mods in 1U rack space. You need to take the faceplates off the mods then slide your mods between the rails. An allen key and bolts are included to secure the mods to the rails.
You can put the faceplates back on if you want, just note they overhang slightly so won't be compatible with all rack setups.
If you're not into soldering, this is a great solution for rack inputs and outputs.