If you’re a traditional fighting game player, you’ll have basic knowledge of what a cross-up is. In the Super Smash Bros. series, cross-ups work a little differently as shields cover a character’s whole body. Rather than messing up your opponent’s block direction, cross-ups in Smash try to make your opponent drop their shield so you can start a combo.
In their continuing mission to inform Smash 4 players, the Beefy Smash Doods have made a video which shows all the ways a player can cross-up their opponent. These include using attacks which can cause a player to pass through an opponent’s shield, attacks which push the opponent in the opposite direction even when shielding, and aerial attacks which knock their opponent in a number of different directions.
The latter category of attacks are called “DI Cross-ups”, as they can cause your opponent to DI in the wrong direction and die earlier than expected.
Source: Beefy Smash Doods