Shield Slash is one of Captain America and U.S. Agent's special moves where Captain America and U.S. Agent throw their shield at the opponent. Light Punch aims low, Medium Punch aims straight, and Heavy Punch aims high. The input for Shield Slash is a forward quarter circle motion and a punch button or attack button depending on the game.
Prior Marvel vs. Capcom 3, Shield Slash, when hits the wall or opponent can only be used when Captain America pick his shield quickly.
As of Marvel vs. Capcom 3, Shield Slash can now automatically returns to Captain America similar to its Power Stone version damage property on the shield's auto return to Cap from Marvel Super Heroes. Additionally, the move's shield throw can now get through the opponent and hits behind the opponent's back during the shield's auto return to Cap. However, the Shield Slash's projectile range becomes very limited, such as while using this move during a close-range/corner combo requires to either cancel to Hyper Combo or perform a combo with Captain America's partner via assist while during normal attack combos before performing Shield Slash ((U)MvC3 only)/Switch Combo (MvC:I).
In MvC:I, there is a variation called Trick Shield, L version is similar to LK Shield Slash and HK version is an anti-air.