A Lockdown occurs when the player misses a Roll Call during the daily routine. The player is then given 98 seconds to escape the prison before .
When the backup timer reaches 0 seconds, you are immediately sent to solitary and items you hid that are contraband most likely are taken away.
It seems that if spotted by a guard, the backup is called off.
When lockdown starts your heat skyrockets to 99% and remains up there until lockdown is called off.
A Lockdown occurs when you activate Prison Takeover mode, caused by knocking out two guards simultaneously.