Focus Attack is Ryu's Down Special Move. Ryu enters a defensive stance and concentrates his strength in his fist. The move can be charged; if charged for a full second, or if another button is pressed, Ryu unleashes a powerful forward punch while a black ink effect shows up (similar to the Street Fighter IV series), stunning the opponent for a follow-up attack.
Overview
If Ryu flashes, this means that the move will cause stun, successfully hitting it causes the opponent to crumple and it goes through shield. It can be canceled by pressing left or right, from charging to even the punch. Ryu has Super Armor during the whole move, but it can only be blocked once, and is vulnerable to grabs.
Origin
Focus Attack, known as "Saving Attack" in Japan, is a universal mechanic first introduced in Street Fighter IV and used by every character in the game. Performed with both the Medium Punch and Medium Kick buttons pressed simultaneously, the Focus Attack grants the user a brief moment of "super armor" with which they can ignore a single attack (although they still lose a certain amount of regenerable health and certain moves will immediately break this super armor.) The Focus Attack can be charged for a second, and when half or fully charged, will force the opponent into a "crumple" state where they slowly fall to the ground, leaving them open to additional attacks.
The Focus Attack button input can also be used during certain normal or special moves, allowing the user to immediately cancel the animation of their move and go into a Focus Attack for additional damage at the cost of a portion of their Super Combo gauge. Furthermore the player can cancel into a Focus Attack, then cancel that move into a forward dash allowing for more combo opportunities including connecting an Ultra Combo.
Ultra Street Fighter IV introduces the "Red Focus", a stronger variant of the Focus Attack that costs two stocks from the user's "Super Combo" meter but gives them infinite super armor to resist hits with (although armor breaking moves and throws still circumvent it.) Dash Cancels and the like can also be performed using the Red Focus Attack.
In addition to Smash Bros., the Focus Attack has appeared outside of the Street Fighter series in Marvel vs. Capcom 3: Fate of Two Worlds as a special attack for C. Viper and in Project X Zone, where Ryu and Chun-Li use it.