When connecting a pit switch for your VTX, the pit switch signal line needs to be connected to TX4 while the power and ground for the pit switch can come from any free VBATT/5V and GND pin/pad on the FC/ESC/PDB. The power will depend on which VTX you have. Please check to see if you need 5V or VBatt. You must also solder the TX4 solder jumper to Normal. To set this up in the OSD you will need to use the Pin Toggle option. When it asks to set your switch to OFF, that is asking you to put the switch in the position you would like your VTX to be off.
A pit switch is an add on device that will allow you to totally depower a VTX from your radio. This is used in team racing as well as in hot climates to keep the VTX cool why sitting on the start blocks, or stuck in a tree!