With a biondo box the SLE is either activated (pedal to the floor) when the TB is pushed wehn you are staged with timers starting at release of the TB button or it could be set using a 2nd push button (can be the bump up/down button) when prestaged that way you are staging with your foot already to the floor at your preset SLE idle rpm and the timers will start with the release of the tb button.
You really need to talk to the seller to see how it is currently working. If you have the Biondo box, you can go to their website and download the instructions.
Since the SLE and delay box racing are new to you, I highly recommend that you get the car ready to race. Make sure there is no fuel in the tank or removed the fuel line to the carb so you do not flood the car and sit in the car, get use to pushing the button and feeling how the SLE is going to work.