Tiger claw strike
Toraken-uchi (literally: “tiger-fist strike”) is commonly called a tiger claw strike, because it involves you imaging that you have a tiger's paw for a hand, allowing you to claw your opponent’s eyes out.
To perform a tiger claw strike, splay your fingers out, bending them at the second and third knuckles, such that they resemble a tiger’s claws, like in the picture below:
[photos of a hand in a tiger claw from the front and the side]
Place the tips of your index and ring fingers on the center of the opponent’s eyebrows, then snap your elbow into your chest, and snap your wrist forward. The end result is somewhere between an effeminate hand gesture and a downward backfist strike, which will rake and pierce the opponent’s eyes with your fingertips. The index and ring fingers will do the majority of the damage; the other fingers serve a contingency plan to cause ocular trauma if the opponent turns away to avoid the strike.
Tiger claw strikes are often setup with rising palmheel strikes; the tiger claw is a “bonus technique” which is spontaneously included to enhance and augment a waza, just like how a grace notes are used in jazz performances. Tiger claw strikes from the side are uncommon, but cromulent.
Because the tiger claw strike targets the eyes, it is forbidden to use in kumite. The tiger claw strike is typically reserved for advanced students, not because it is difficult, but because there are more important things that up-and-coming karateka should be worried about.