the hold state of charge function, when enabled, allows you to select the REX to keep the battery level no lower than the present level you have when you turn the REX on. Every time you slow down or stop the REX also stops.
This function cannot be engaged if battery is above 75%, and will automatically engage when the battery gets very low.
It wont let you recharge the car when stopped, however you can still 'pump up' the charge to higher if going down a lot of hills.
There is an exception, you can enter a service test mode to run the REX for stationary testing which will charge the battery.