A throttle body closes gradually to control the manifold pressure of an SI engine.
What you probably have is a safety device called a shutoff. When you pull the cable, a blade or butterfly snaps shut, completely closing off the the Diesel's air supply.
Cutting off the air suppply is the only way to save a runaway Diesel from destroying itself. A runaway doesn't happen often, but it does happen. Diesels normally run with the air supply wide open, and speed is controlled by metering the amount of fuel oil that's injected. Unfortunately, Diesels will also happily run on their own lube oil, e.g. when a piston ring fails. Shutting off the fuel supply won't stop the engine if it's running on lube oil.
I personally think shutoffs should be required on all Diesels, but most manufacturers would prefer to saddle the owner with the cost of a runaway, and pretend it never happens to their engines.
Mike Halloran
Pembroke Pines, FL, USA