Lock up switch?

[czarcaustic]

Quick question for the minds of the board ...

I just bought my lil' '72 GMC Short Fleet. The PO had placed a 350sb and some unknown (to me) automatic transmission in it. To this point the most I have ever done to any vehicle of mine is change the oil and replace a battery. Now I have a whole new wonderful greasy world to explore and I am a bit overwhelmed.

My truck has a little nondescript toggle switch intalled in the dash ... I have flipped it while idle and while the engine was off and nothing changes. The widow of the PO I bought it from didn't know what it was. I was talking to an old timer here in town and he said it was most likely a "lock up switch." Unfortunately our conversation was only in passing and I was unable to ask him what it was, what it did, and if it was necessary.

Can anyone explain what a lock-up switch is and if I need it? Furthermore, If it isn't a lock up switch, any ideas of what it could be.