I have seen that before but they didn't use a real finger on the video I saw
If this is the one I'm thinking of, there is good reason for that. Despite how fast the saw actually stops, it still must come in contact before it will send the signal to stop the blade. This means that you're getting cut no matter what. It may just be a scratch but it won't completely save you.
Now, would you volunteer to use your finger for the demonstration knowing that you're still going to get cut a little? Would you demonstrate it thousands of times for various videos and promotions?
I don't know many people that would.
Also, I don't think this would be nearly as effective in a chainsaw kick-back scenario.
Why?
If you slow the video down, you will likely see that the blade still rotates quite a bit before it comes to a stop. This means there would still be lots of cutting going on if the finger was being fed into the saw very quickly (much like a kickback happens).
Will it help in kickback? Most likely yes, but you're probably still going to go get stitches.
Also, because the "stop signal" is a very, very fine electrical signal, there must be excellent contact throughout the system. That just simply isn't possible on a chainsaw chain. The electrical signal would have to pass from link to link, and also from link to bar, and bar to electrical contactor. First off, each of those contact points are going to be insulated with oil and debri. Even if that weren't the case it would just be impossible to ensure that all contact points remained connected, there are just too many to keep track off on a constantly changing system. I think putting a system like this on a saw would develop complacent behavior and create a false sense of security.
But that's just my opinion....