As Colin has already stated, the switch setting at the bottom of the mix has 2 positions
which allows you to activate the mix in either or both of the switch positions. Also,
Unless you have a DX18QQ, you don't have origin mixing which means that when you
mix elevator to your brake control channel that you are mixing the output of the elevator dual rates
and that depending on your dual rate settings you will get different braking behavior.
If that is what you want, then you are all set, if you want to have your mix behave exactly
the same regardless of dual rates / flight mode, you need to get a bit creative in the channel assignment
menu. Since I didn't want the dual rate setting to effect my brakes and since I'm using a 12 channel receiver
I did the following:
1) Enable X-Plus mode so that XPlus1 (channel 11) and XPlus2 (channel 12) are active.
2) In Channel Assignement X-Plus Input Config menu I assigned Ele to X + 8, which I'm NOT using.
3) I then created a mix which mixed X+8 -> Brake control channel and used the Gear Down position to activate the mix.

If you are using an XPlus expander module and are using all 18 channels then you can't get away with this work around.
Hopefully, Origin mixing will be added to the regular DX18 with the next software release but in the mean time, this is
how I worked around the issue on my Boomerang.