Lockdown (独房送り, Dokubou okuri)? is a mechanic in Persona 5.
Lockdown is unlocked when the Strength Confidant reaches rank 3. It allows a Persona to stay in the Velvet Room for training new passive skills for covering its own weaknesses. The time for training completion depends on the Confidant Rank of the Persona's corresponding Arcana, shortening with increased rank; when uninitiated it takes 5 in-game days, while at max rank it only takes 2 days.
While a Persona is in lockdown, it cannot be used in battle, gain experience or be used for fusion until the protagonist retrieves it. If it remains in lockdown for more than 10 in-game days, the Persona is lost. However, Caroline and Justine will inform the protagonist via their phone when they reach the deadline for releasing their Persona. Accessing the Velvet Room doesn't consume a time slot, so the player has nothing to lose in collecting their Persona, but be careful if plot-important events forcibly advance time past their deadline.
The skill learned from lockdown will, most of the time, be specified to cover a weakness. For instance, a Decarabia is more likely to learn Dodge Phys or Resist Phys than other resistance passives. If a Persona has multiple weaknesses, the skill it learns will select a random weakness to cover. Rarely, you may get Dodge/Resist passives of elements that are not the Persona's weakness, or even passives that resist ailments; raising the rank of the corresponding Confidant will reduce the chance of this happening.
If a Persona already has a resistance/evasion passive, even if said skill isn't covering its weakness, the passive it gains from Lockdown will be randomly determined, up to and including ailment resistance passives. This also applies if Lockdown is used on Personas without any innate weakness. The type of skill to be gained is decided once the Persona commences its detention; if the Persona picks up the wrong skill, reloading to just before release will not change it.
At Strength Confidant rank 8, Personas left in Lockdown will begin to learn Null and Evade passives.