ALL CREDITS TO DR.M OVER ON HELIFREAKS.
How to set up your DX6i to use all 3 banks (setups) on the Brain 
First, use the new setting within the Brain software to lock the tail gain: Common>Receiver> Gain: click the check box "Is set in software"
Then configure "Setup Selection" to use Channel 5.
Go thru the 3 "Setups" and click the "Heading Lock" check box if you want to fly in HH mode all the time. I set my 3rd bank to use Rate mode for the tail.
Tx Setup:
Go to the Dx6i Gyro menu and set the rates to:
0: 30% (Setup1 on the Brain)
1: 64% (Setup 2 on the Brain)
As you know, the DX6i does not have any 3-position switches that you can use directly for switching the 3 flight banks. We'll be using a Mix to accomplish this. Go to the DX6i menu and select Mix1. Activate the mix and set it up to
mix Gyro>Gyro
Set the mixing ratios to:
D: -0% U:+125%
Set the switch to one of your free switches (I used Ail D/R).
The way this will function now is:
Ail D/R position 0
- Gyro switch (Gear) position 0: Setup 1 on the Brain
- Gyro switch (Gear) position 1: Setup 2 on the Brain
Ail D/R position 1
- Gyro switch (Gear) position 0: Setup 1 on the Brain
- Gyro switch (Gear) position 1: Setup 3 on the Brain
Verify this by looking at the blinking LED on the Brain, or by connecting the Brain to the software and watching the Receiver tab.
As an example, I'm using the 3 banks as follows:
Setup 1: general flying (Tail in HH)
Setup 2: Rescue mode with SL (Tail in HH)
Setup 3: Autorotation mode with low cyclic gains, tail in Rate mode, and governor bailout.