I hate spreading out my favorite little house rule (Like giving away a culinary secret) but I thought it would be funny to see some general responses on this. Me and a few members of my group made the rules for Jackpot. Why? Because in our last campaign the uber-powerful magic user was finished off by the move Jackpot from DMC. Anyway, here are the rules:
To learn Jackpot you must be taught it from somebody who already knows it. Learning Jackpot requires ten karma. Jackpot is not it's own separate skill, it attaches to another skill such as pistols. If you have Pistols 7 it would then become Jackpot Pistols 7. You can still, of course, use pistols as it's own skill. Jackpot can be attached to any offensive skill, including spell casting. Exceptions include Conjuring and....That's about it really.
Jackpot can only be used when at least one other person is around with Jackpot, they don't have to match the attached skill, all it requires is that they have the Jackpot skill. When used, all members (Up to five) of the attack roll normally but then combine their successes into one group of net successes against one opponent. At this point, temporarily reduce the users (Yes, all the users involved) Karma Pool/Edge to 0 until the next session (As opposed to the next in game day). Karma/Edge can not be used on the roll itself.
Jackpot can only be used against enemies considered Primary Runners or just general big baddies, such as "Bosses." It can only be used when someone else with the skill is around and only once per session. However, against "Unbeatable" foes such as great dragons it may be used more then once to strike the fear of impossibility into your players hearts. Jackpot can ONLY be taught, you cannot learn it on your own or without instruction.
Explanations: Jackpot really can't be explained. If you asked somebody who knew it they would probably say it's just something that feels natural, as if your in synch with the other person, lining up perfectly for an attack. It feels almost like a side effect of adrenaline and fear.
Notes: Jackpot was made to be a finisher and is supposed to be cinematic. You should not allow PCs to use jackpot with NPCs. It's best, in fact, if you severally limit the amount of NPCs who know the skill. When Jackpot is used try to describe it in detail and in a way to impress your players. If they use Jackpot successfully but don't kill or defeat the target, try to strike fear in their hearts with the realization that their best attack failed, leaving them without karma and lower on options.
Example of use(Statistics only, not roleplay wise): The group decides to Jackpot the final baddie of the entire campaign. Joe Decker has Jackpot Pistols 7, MageWiz has Jackpot Spellcasting 6, Swordman Adept has Jackpot Sword 10, and Hotshot has Jackpot Rifle 8. They can either roll their own dice separably or the GM roll all of them for a total of 31 dice. Combine the total successes together. Assuming Joe gets 4, Made gets 2, Swordsman gets 9, and Hotshot gets all eight, that's 23 total successes. Then combine the damage code together however you please. Is this overpowered? Yes. But the hit comes when their karma pool/edge gets cut to zero and the fact it can only be used on specific foes. Not to mention, if the runners get split off from each other they can't use it an, ultimately, the entire group needs to cough up the ten karma to learn it for it to be effective.