The casino industry’s first tentative step toward skill-based gaming was to simply slap arcade bonus rounds into standard slot machines. You play a normal video slot, but once you either get the right combination of bonus symbols or collect enough of them to fill a meter, you go into a bonus round where you play the vidya game. Your score in the bonus round determines how much money you win.
A number of major slot manufacturers were perfectly positioned for this sort of thing, because they’re former arcade game developers that gradually transitioned to gambling over time … for example, Konami and Bally (now a subsidiary of Scientific Games).
The way you would expect the developer to structure the game to retain the house edge is to make the maximum bonus round payout still be low enough that the return to player (RTP) over the life of the game is still somewhere south of 100%. So it kind of becomes like blackjack or video poker – the player’s skill at the bonus round can increase the RTP somewhat, but even if they play as well as is humanly possible they still expect to lose money over time to the house edge.
That’s not necessarily always a bad thing, though. The way many people make money off of casinos is to play games that have a high but still negative RTP, like 98 to 99%, and then make up the difference in losses through the comps the casino gives them. Slots are usually the most heavily comped game in the casino, because they make the casino the most money and are among the most short-term volatile and long-term unfavorable to the player. If a skill-based slot is comped like a 90% RTP slot but you can get the actual RTP up to 98% with skilled play, that could easily create a positive expectation situation in terms of comp return.
The other moneymaking possibility here is exploits. These usually aren’t illegal, but casinos may ask you to leave the property and not come back if they suspect you of pursuing them. Most slot exploits involve somehow recognizing when the machine is close to entering a bonus round (if it is on some sort of a schedule), hopping on and playing through the bonus win, then taking off and looking for another “primed” machine. You usually won’t find slot exploits by Googling, though, especially for newer games – people who figure these things out usually guard them carefully, as once they get widespread enough casinos start either software patching exploits out or simply removing those games from the floor. Exploits are generally found by experimenting with games yourself, or going to a casino floor and looking for certain signs of sharp slot play – a bunch of people spending a lot of time on one game type, getting shifty and protective of their seat if someone comes around and starts watching them.
Here are the slots with skill-based bonus rounds currently out there in the wild: