Way to unlock: You need to complete main mission 73 (Meltdown). Wait for a phone call from Jimmy, answer it and listen to the conversation.
Playable characters: Michael
Additional information: This one is a quite unique mission because it is optional and it does not count while calculating the current progress in the main plot. The premise of this one is that you help Jimmy who has got into trouble, of his own fault.
Some time after the completion of mission 73, you should receive a phone call from Jimmy, while playing as Michael. During the conversation, you will learn that Jimmy was kidnapped by the persons that he offended online. At the very end of the conversation, you need to decide whether you want to help him or not, and it is a better option, of course, to select the former.
The car that Jimmy is being held in is traveling across the wilderness in the central part of the map (Banham Canyon). After you come close to the SUV, start firing any weapon at it but, in accordance with the instructions that you get on the screen, try not to kill any of the abductors. After the car is sufficiently damaged, it should stop and the abductors will start running. Ignore them and, instead, return with Jimmy to the current living place of Michael's family.
Rewards for the completion of the mission: none
Requirements for the completion of the mission in 100%: There are no additional requirements for this mission so, it gets completed in 100% automatically.