Hi Jon,
If conditions are right then you can force EV mode yourself rather than leaving the cars algorithm to control it.
In the CT loaner I had the EV button seemed like a bit of a gimmick as the button override only worked on a few occasions.
The vehicle needs to ensure that the battery is being managed correctly, the cabin heat is sufficient, etc so often the engine keeps running.
In my older hybrid, there isn't the option to force EV mode via a button so there's less to worry about 😉 Although in my old barge, you can force EV mode with your right foot (providing x, y, z all pan out).
Cheers, Lee