hot-stage

verb1

1. •
   To light the engine(s) of a rocket stage while still attached to the stage beneath it.

   • The Proton's second stage avoids ullage issues by hot-staging, allowing the engines to ignite while still being pushed by the first stage, but this necessitates the use of a special open trusswork in the interstage area to prevent the superheated second-stage exhaust from blowing up the rocket.