The Iki spike is hollow so when you spike the brain from the front, you would have located the nerve canal.  Leave the spike in place and simply push the nerve-killing wire into the spike and into the nerve canal.  There are plenty of informative Youtube video on how and why. 

There is a Super Long set especially for Tuna and Kingfish.  Learn how to do this process and you will be well rewarded with better eating and longer lasting fish.