A carry over from Super Smash Bros. Brawl, some characters in Smash 4 can suffer increased landing lag after being knocked out of certain special moves. Termed “Recovery Carryover Lag” or RCO Lag for short, this engine quirk can leave characters open for a punish after having a move interrupted.
Initially documented in Smash 4 by ESAM, the Beefy Smash Doods have taken their magnifying glass to RCO Lag to explain how players can potentially use this extra landing recovery to queue up a specific punish. RCO Lag is actually split into two separate divisions: Soft and Hard RCO Lag. If a character does not fastfall from their interrupted special move, or falls from a low height, they will undergo Soft RCO Lag. If you fastfall however, you will experienced Hard RCO Lag, which is significantly longer that the Soft variant.
There are ways to cancel the RCO Lag, mainly by either teching the ground, grabbing a ledge, or landing with an aerial. However, landing a tech or trying to autocancel an aerial will involve you suffering RCO Lag, so just be aware of that when trying to return to stage. You can also effectively “store” RCO Lag if you land with a special move like Fox or Falco’s Phantasm, meaning you’ll receive the extra recovery upon your next landing. However, you can cancel this stored lag by performing any of the previously mentioned cancel methods.
Sources: Smashboards; Beefy Smash Doods