There are three multiball modes in High Roller Casino. They are all 4-ball multiballs, each with fairly generous ball savers. They are not stackable.
Slot Machine Multiball
Slot machine multiball is the multiball mode that is played most often. After each time the slots are played, the ramp goes down all the way, and the next slot machine shot will lock the ball. Locked balls are held on the wireform ramp just above the right flipper. When two balls are locked and the ball is shot into the slot machine with the multiball lit, Slot Machine Multiball starts. This mode may also be initiated from the Roll-n-Win wheel.
Once started, the goal is to shoot as many of the major shots three times before shooting the slot machine for the third time. For each of the major shots (main ramp, left orbit, right orbit, slot machine), the first time it is achieved scores a "jackpot", then a "double jackpot" on the second occurance, and a "triple jackpot" the third time. For each of the first three shots you can get to triple before shooting the third slot, you will achieve a different 'level' of super jackpot.
If you have:
No other triples: Super Jackpot
One other triple: Super Duper Jackpot
Two other triples: Super Duper Mega Jackpot
All three triples: Super Duper Mega Extreme Jackpot
Obviously, each level of jackpot is better than the previous, and the last one (very difficult to get) is worth quite a bit of points.
Slot machine multiball is typically played several times per game. My personal experience is that Slot Machine multiball isn't usually terribly significant, but if you can manage a Super Duper Mega or Super Duper Mega Extreme Jackpot, it can be quite significant.
When all six of the casino games are won, Casino Frenzy is lit. Either orbit or the main ramp will start the mode. Shooting the slot machine is supposed to start it too, but there appears to be a bug in the software that simply releases the ball to the right flipper.
When the mode starts, all four balls are put into play, and the goal is to shoot each of the casino games once. Poker and Hi-Lo are both achieved by a single shot initially. Once all games are successfully made (each scoring a lesser jackpot amount, starting at 10 M), the center ramp will be lit for Casino Jackpot. Then you start the whole process over again, but each game must be 'qualified' for in order to spot it; i.e., a poker hand must be drawn, the slot machine must be 'bashed', etc. Casino Jackpot is 100 M points the first time made, and it increases by 25 M each time thereafter.
Break The Bank
Break the Bank can be the most lucrative of the multiball modes, but it can also be the most frustrating. When 100 chips are scored, Break the Bank is lit. You cannot simply ignore the mode and keep playing; the game will not award any more chips until this mode is completed. Once lit, it is started by shooting any of the major shots, as indicated by the large white flashing arrows.
The goal is to continue to shoot the four major shots as many times as possible before shooting a ball through the slot machine with the ramp down. The slot machine ramp is constantly moving up and down, but stays in the lowered position for only a few seconds at a time, then raises again for a much longer interval. The more times you can make the other shots, the higher your Break the Bank Jackpot will be when you finally manage to shoot it through the slot machine. At that point, you must make all four of the major shots, each of which will score the entire Break the Bank Jackpot, before you will be allowed to continue and build up the jackpot again. If you shoot it through the slot right away without achieving any other shot, the Break the Bank Jackpot is only 25 M, but it can get very high. I've seen it in the neighborhood of 200 M each, which would yield 1 B points if all of the BtB Jackpots are made.
The frustrating part of this mode is that sometimes it seems that the ramp is never lowered at the right time.