The dataref is spring-loaded. You have to keep setting it.
Note that this dataref is meant for __authors__ who are using generic instruments or animations and need a single dataref with which to build an entire ignition.
If you are using the SDK in a plugin, there is no need to use this dataref; simply use the starter commands using the XPLMUtilities command APIs. The command API lets you run a command until you say it is over. Use sim/cockpit2/engine/actuators/ignition_on instead.