My late 77 has the typical mechanical kick down linkage from carb to trans.On another website someone is talking about an electrical switch under gas pedal that does the same thing as the cable type. Can someone explain?

A '77 should have the 400 trans, which would have the electrical "kick-down" set-up. The 350 trans wasn't installed in Corvettes until '78...I think.
So....if your car has the 350 trans, with a cable from the carb to the pass side/front of the trans, the cable controls the kickdown, or passing gear. It will also hold the trans in a lower gear under heavy throttle.
The 400 trans uses an electrical solenoid, activated by a switch mounted to the accelerator pedal bracket. It also only controls the kickdown, and will hold the trans in a lower gear under heavy throttle.
There are a couple of other threads here about the kickdown switch, and the adjustment procedure, with some pics.

I have a Late 77 My vehicle invoive shows it came with an M40 is that a 350 or 400. When I purchased the vehicle there was no passing gear and there was no kickdown sw above the gas pedal, I had to run the wires from the trany to the sw, install the switch. I have posted on this before. i cannot get the switch to work properly, you push the gas pedal and it will push the sw to far and only work once. I ts just getting to hard to stand on my head and get to the switch get it fixed. 

Dad's 76(it is a late 76 L48) vette came with a 350turbo, has the mechanical linkage, never a switch under the gas pedal...

This may get interesting!
M40 was the rpo for the 400 trans, and the rpo code for the 350 was MX1.
Going from the info I have, the rpo code changed from M40 in '77 to MX1 in '78. My assumption was that they did the change sometime in the LATE '77 model year.
For the sake of the original question, it's a moot point. But....this is the kind of stuff that makes doing research so much fun!

My description of how the two different systems work is correct, tho, so whichever trans you have, that's how it works.

So, you have one or the other.My factory 350 trans,again a late 77, has the mechanical linkage I talked about when starting this post. Never was any wires hanging out it's butt,and no sign of a switch near the pedal. Hope by asking the question, someone else learned something as well. Thanks again to all. Two mos. to go....2 1/2 yrs. total on the re-do

This is one of the area my my 77 that is confusing because I am not sure that my tany is original. when purchased by me it did not have a kickdown mechanical or electric. and the linkage hookup at the trany doesnot look original. I think I have a picture but the bracket at the trany is attached by just one bolt of the trany pan. I recently sent for the factory invoice which shows it came with the M40 original, but I beleive the engine and trany are not the original ones. I quess I need to look into this more, i wanted to get the kickdown working mostly so everything works but it is getting harder physically to get under there to solve the issue with the se I need a trunk munkey to help

My apologies to all following my post here.My trans IS a 400 and not a 350 as I stated. Yes it for sure as a mechanical kickdown at carb.

This may help answer my delima. are some 400 cable kickdown and some electric?.