We used a tool similar than the one here, it was made from reinforced steel rod, the point sharpened for penetration.
The tuna have a soft spot in the middle of their heads (most fish have the same)
There is a method to bleed the fish prior to using the 'kill' method...prick the fish under his pectoral fins, close to the bone (theres a certain spot) prick the fish between his pelvic fins (dead center, theres an artery there too)
With the fish now on his side, cover the eye with your hand hold the gill plate and drive the spike into his 'brain' at 45 degrees to the lateral line, holding onto the fish.
This will effectively touch the nerve and stun the fish to silence, allowing the blood to exude from the 'pricks' made in his flesh. the fish will flutter and kick his last breath and will be left motionless, mouth wide open, bleeding.
At times with bigger Big Eye Tuna the fish is 'stroked' from head to tail, allowing more blood to escape the body.
A good time to bend over and get a mouthful of fresh Tuna blood
Note this method particularly used on Big Eye and Black-fin.....as their flesh is more valuable than the Yellow fin which is graded as a tougher, drier meat and has little monetary value to the Japanese Sashimi market.
http://www.diffen.com/difference/Sashimi_vs_Sushi