Pillaring refers to pressuring an opponent's shield in Super Smash Bros. Melee by utilizing a combination of techniques (SHFFLs, aerials, and shines) performed by Fox and Falco. This is an advanced technique commonly used in high-level play. The term pillaring is also sometimes used to describe pressuring shields in general. It is not to be confused with Falco's other down aerial and shine combos.
Pillaring can lead to a defending player rolling out of their shield and being chased or having their shield broken, although the latter is rare in high-level play. Pressure from pillaring is usually relieved by a successfully performed shield grab or out of shield option, such as Samus's Screw Attack, Bowser's Whirling Fortress, or even Fox's shine. Characters with poor out of shield options are especially vulnerable because a successful shield grab requires very strict timing.
Pillaring by Fox and Falco
Pillaring is performed by SHFFL a down aerial and immediately shining. The shine is then jump-cancelled into another SHFFL'd down aerial, and the cycle continues. Grabs and other aerials can also be cancelled into in place of the down aerial. This technique is sometimes used in tandem with waveshines and waveshine infinites and followed up with jabs or down tilts.