A sleeper troll is one who has infiltrated into the target online community and 'gone to sleep', sometimes for many months even years.
Often times the sleeper troll may appear to be in general agreement and in group solidarity of belief or sentiment with the targeted online community.
It is usually only after the targeted online community has suffered some form of set back or upset that the sleeper troll shows it's true idenity as a troll pouncing on the opportunity with the intent of provoking other users into an emotional response or of otherwise disrupting normal on-topic discussion.
Troll operatives can be used in varied forms. Hidding behind internet pseudonyms, a troll could be someone employed by political operatives, attorneys in civil letigations, civil rights cases, workers compensation cases, social security case, etc.
A common practice of the troll is the 'gaslighting' technigue:
Gaslighting is a form of intimidation or psychological abuse in which false information is presented to the victim, making them doubt their own memory and perception.
Sleeper trolls pretend to be part of an online community and in time show their true idenity as a troll by disrupting the community.