The selector plate is the small plastic plate on the side of the gearbox. When turning the selector on the side of the gun from semi to auto or safe, the selector plate is the part that changes the internals of the gearbox. But there are improvements to be done here.
On most guns the selector plate has a small cobber band on the end. Underneath the selector plate, there's 2 small cobber connection tabs sticking up, touching the cobber of the plate. These cobber tabs are from the internal trigger switch. When the selector plate is in safe, these tabs can't hit the plate and connection is broken. And when in semi or full auto the connection is made.
The problem with this way is that every connection in a gun creates electrical resistance. And that decreases amps. (U=R*I or I=U/R) Which translates into higher resistance gives fewer amps at a given voltage. So instead of using the selector plate to close the circuit, you should solder a peace of thick wire between the tabs under the plate. This will give less resistance and allow the motor to pull more amps.
This will destroy the electrical safety feature in the gun. Because now connections are not broken when in safe. But almost all gearboxes have an extra safety. Inside the gearbox there's a small plastic peace that (when in safe mode) block the trigger from moving. So instead of 2 safety features there’s only one. But hey why have 2 when one is enough? Especially when you can increase performance!
Well most of the how has already been described so really all you need is to see a picture. But also know that if the new installed wire block the selector you can cut off a peace of the selector plate and remove the cobber strip to get to fit.
Doing this mod, is simple, and the benefit may be small. But it largely depends on the rest of the gun! I have seen guns where resistance was the biggest enemy E.g. my personal Mp5K running 120 spring, on 13:1 speed gears and a GnP 170 devil jet motor. This gun went from 27 to 33 rounds per second on a 7.4v Lipo just by doing this mod because I am drawing a lot of amps to pull the heavy spring on the heavy gears. At the same time standard guns gain near zero increase.
So doing this or not, depends on where the limit of your gun is... One question: Do you have a fuse in your gun? If yes, then start by removing the fuse first! Because it's resistance is bigger than the selector plate resistance. (Be aware that removing the fuse removes the safety protecting the electrical system and motor). Personaly i recomend the mod on all guns, because any performance increase is nice right?.
Besides doing the sector plate mod what else should I do to my electric system? Well if you are looking for maximum performance these things will all reduce resistance: 1. Remove the fuse. 2. Get thicker gauge wire (more strands and thick cobber). 3. Replace the connectors to E.g. Dean connectors. And 4 If you are really wild rewire using a Mosfet! Check out my Mosfet guide for details.