Afternoon Tiger, contrary to its appearance, is purely taijutsu in nature and is nothing more than an incredibly fast punch as Might Guy has noted. After opening the seventh of the Eight Gates, the user places a palm facing forward in front of their face with one hand and then taps it with his other hand, formed into a fist, which creates a massive amount of air pressure. Next, the user forms a unique hand seal resembling a tiger, which launches the air pressure at the opponent in the shape of a tiger, by leaving a gigantic tiger-shaped impression into the initially built-up air pressure with the hand seal. The air pressure will condense as it's moving and be focused into a single point. The technique then explodes on command, releasing the built up air pressure in an instant. A massive concussive explosion is created which can be felt a significant distance away.
The unique hand seal used in this technique resembles the "Tō" (闘) seal, also known as the "seal of the outer lion" which is used in the practice of Kuji-in.
In chapter 608, Guy was seemingly able to perform this technique without opening any of the gates. It should be noted that the attack seemed significantly smaller than in his use against Kisame Hoshigaki. Doing so however greatly drains his chakra, suggesting that opening the gates allow the user to perform the Afternoon Tiger without significant chakra loss.