depending on your computer code, your TC will lock up as low as second gear, and STAY LOCKED even while shifting to higher gears.
The link above to the homebrew TC lockup has two LEDS. One for letting you know what the COMPUTER is doing, and one to let you know when the manual switch is on.
I have never messed with my manual lockup switch at the track, because my computer detects WIDE OPEN THROTTLE as a really good reason to lock up the TC as soon as possible. Typically during the 1-2 shift. It stays locked the rest of the track until I hit the brakes at the end of the track.
I use the manual TC LOCKUP mostly for towing. I use it for keeping the TC locked during downhill grades. When I tap my brakes, the TC unlocks, and I lose what little engine braking I had.
There is little to no racing advantage to the TC lockup mods. Most of the computer codes I have ridden with lockup fast enough, however even having JUST the indicator LED, without the switch, will tell you lots about what your tranny is doing.