If you fire on a jammed or cloaked ship with a weapon using an EMP, Push, Pull, or Stop effect it un-jams and/or de-cloaks the ship.
This happens regardless of the size of the weapon or damage done so long as it has one of those effects that pass through shields.
[[ https://starmadedock.net/threads/emp-un-jamming-ships.29080/ | HERE ]] is a current forum thread on the topic where it has been confirmed by several other players.