JCotte,
What is a mystery switch and its function? Good question.
The pcm controlls the tc lockup clutch solenoid . By gounding the solenoid it basically locks up the tc clutch inside the torque converter.
It does so when it receives the signal from the throttle position sensor, and vehicle speed.
By installing a manual switch, you are simply bypassing the pcm by supplying your own signal to the lockup solenoid.
A torque loc basically does the same thing, with the exception of it automatically dis-engages the lockup at a preset speed usually 30 mph .
These devices sound pretty hamless dont they?
Its not until you actually understand what is happening inside the torque converter and transmission by using these devices do you realize what the concequences are, and by then its usually too late.
I used to get pretty worked up every time the subject of the the torque loc came up, you guys that are new can do a search under it, some of the info is actually pretty informative.
If you owned a 5 speed or 6 speed transmission, would you put it in 5th or 6th gear depending on your transmission and leave it there, no matter what, light throttle , heavy throttle,full throttle, uphill, downhill, would you do that to your standard transmission?
If the answer is yes then i would suggest go ahead and get a torque loc, as it forces the same results in the automatic.
In other words, you pucker up and be prepared to kiss your tc goodbye.
The results will be the same whether you have a factory tc or and aftermarket one. The aftermarket tc may hang in a little bit longer but the results will be the same.
By engaging a torque loc or a mystery switch this activates the loc up circuit in the vb, it basically locks the transmission into that one gear. It does not allow any passing gear to happen.
You can verify what i am talking about by using a pressure gauge on the governor port. I can tell you guys that want to verify this how to do it if you want to , you can call me, make sure you have a 0-100 psi pressure gauge with about an 8-10ft hose.
We realized however there was a need to have an exhaust brake system for the automatics for safety reasons, we had to figure out a system designed to protect the transmission and the tc.
Let me warn you guys of this, dont let anybody kid you guys, any system will cause some wear and tear on the transmission.
When we designed the smart controller for the automatics we looked at serveral reasons why the transmissions were failing when using an exhaust brake,
One feature we added to the controller was a tps signal, by adjusting the signal you can unlock the tc at a pre-set throttle position simply put passing gear.
The other feature we put into it was the od circuit.
We wanted our controller to automatically dis-engage od if you dropped below the pre-set speed to keep the rpms high.
We wanted the transmission to be able to automatically gear down as selecting the right gear ratio comming down a hill is just as important when driving a standard.
Ex, if you were slowing down from 60 mph and your vehicle speed hit 50 od would automatically be dis-engaged.
We designed 2 versions of this , one was for city use and the other for highway use.
The city version basically locks out od all the time below a certain speed.
We have also added a 2nd gear loc up feature that disables lockup at a preset speed.
The oem factory vb does not allow 2nd gear lockup in its orignial state.
The reason why we went to such great lengths is because we know the importance of keeping the rpms high, we needed to protect the lock up clutch.
Most any exhaust brake you will find is good quality,
On a standard you can pretty much put any exhaust brake on it and will get good results.
On an automatic i chose the PacBrake because of the excellent retarding hp at the higher rpms and at the lower rpms the Pacbrake does the least amount of tc clutch damage compared to the other brakes on the market.
Your tc clutch at is weakest below 1600 rpms simply becuase of centrifigual force.
Attention: TDR Forum Junkies 
