It is a switch to lock up the converter in either 2nd or 3rd gear while drag racing. You do not use/need it for normal driving as the computer still locks up the converter. What it does is almost acts like an extra gear in the 1/4 mile under WOT because the tranny will not lock up under WOT.
With a high stall converter what you gain on the bottom end, you lose on the top end without a lockup switch. With the stock twins you will lose 3-4mph with a high stall if you do not have the lock up switch. With the lockup switch, you will have improved launching ability, and keep the top end charge (and less slippage/power loss) of the stock converter.
The high stall converter slips (even at high rpm) and puts less load on the motor, so at the same wastegate setting you will tend to get less boost, and put less power to the ground than you would with a stock stall at the same boost level. Typically when you hit the lockup switch you will gain 2-4 pounds of boost without touching anything else, so keep that in mind especially if you have a single running at the edge of octane/fuel system capacity.
Remember to to turn off the switch after your run or you will think your car is broken when you are driving slow in the lower gears.