Single Speed Maxx Questions

[GorillaMaxx360]

My friend is building a maxx and he wants to make it as cheap as possible while still making it reliable. I told him of the single speed and because he is going Brushless he decided to get it. Recently i told him that GORILLAMAXX sold just the coupler for a lot less money. So he decided to keep the stock trans and use just the coupler so he would not have to spend all the money on the single speed conversion. My question for you guys is how does he use the single speed locker/coupler to lock the trans in 2nd gear and still using the stock diff case. What gears to i need to keep in take out and such. Pics, diagrams, and any other items to assist my understanding of this concept would be appreciated. (I am well aware of how to install, use, and know how the regular single speed works i just do not understand how to use the coupler while keeping the regular trans) thanks