Due to the static thrust capability of the hybrid rocket engine, the vehicle can take off under air-breathing mode, much like a conventional turbojet.[3] As the craft ascends and the outside air pressure drops, more and more air is passed into the compressor as the effectiveness of the ram compression drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.
At Mach 5.5 the air-breathing system becomes inefficient and is powered down, replaced by the on-board stored oxygen which allows the engine to accelerate to orbital velocities (around Mach 25).
https://en.wikipedia.org/wiki/SABRE_(rocket_engine)#Engine
Wonder what the un-simplified version looks like... ;)