Are you talking about the Bell 206 helicopter with an Allison engine?
If my memory serves me correctly pressing the start button causes electrical power to enable the starter motor to turn the engine, when a satisfactory rotation speed has been obtained the throttle is advanced beyond the stop to introduce fuel which the iginters should now ignite the fuel/air mixture, the engine continues to accelerate until it reaches a self sustaining speed and stabilises. Opening the start valve without having engine rotation probably won't achieve anything much as the engine driven fuel pump won't be turning, I think!
IGN ENG would be the CB that protects the engine ignition/igniters
and Start Eng would be the CB that protects the starter motor, again, I think!