What is the exact reason that electrical stunning before shechita is forbidden?
Excerpts from sources please if possible.
This is not a definitive answer or the final word but I will translate a responsa from Rav Yosef Eliyahu Henkin printed in chelek 2 of Kisvei haGria Henkin page 50.
'I have postponed responding about stunning with electricity because being far away it's hard to get clarity, and especially as it seems all is not known about this. But, some time ago it was made known from Sao Paulo Brazil, and it was described that they chase after the animals with the electric device and with it they fell the animals and drag them to the slaughterhouse. This was considered as far as the nephila, falling, and because a misukenes needs pirkhus and because of the spine. And now I will escape giving a ruling on this to research this subject, if there is haphala, and if there is pirkhus, and to discover the effect of the electricity on the spine and the brain, ether through doctors or testing.
Seems like the reason is that it may cause it to actually be dying or it may cause internal damage while shekhita thus turning it into "nevela" or "trefa". There we some rabanim who permitted it but they are minority.
One of the most comprehensive source is "Edut Neemana" (hebrewbooks.org/22298). regarding the trefa see page 12 toward the end.