Back in the day, Duke Nukem 3D was the game to play (until Quake came out). This game required a fairly fast computer; The average computer was a 486 and Duke would barely run on it. The minimum spec was a 486 with 8 MB ram but this was too little for serious play, never mind high-resolution 640x480 graphics. A friend of mine and I had similar computers: 486 DX2 66 MHz, mine with 12 MB ram, his with 16 MB. Duke Nukem ran acceptably on my computer; low-res mode was fine but 640x480 was a little choppy. My friend called me one day to complain that Duke Nukem ran very slowly on his computer. We talked about RAM, using MS-DOS boot-disks to eliminate TSRs, HiMem.sys, and lots of other esoteric things, knowledge of which was de rigueur when dealing with DOS and memory-hungry games. We concluded that it wasn't the RAM and I suggested he bring the PC to my place. He did, and we disconnected my PC and plugged his into my monitor, keyboard, mouse, etc, and started the game.
It was like watching Duke Nukem, only as a slide-show. The game was truly unplayable and this was surprising considering how well it ran on my computer. I checked his settings and could find nothing wrong with the setup. Then, on a whim, I pressed the seldom-used button on the front of the case.
Back in the day computers weren't fast. The original IBM PC was a blazing 4.77 MHz. Eventually new computers were released with faster processors and RAM. However there was a large number of programs (mainly games) that relied on the timing characteristics of the PC: the games didn't had a timing loop that was tightly coupled to the speed of the processor. Run the game on a faster processor and the whole game goes faster. The aliens attack more quickly, the flight-simulator flies faster, and the ghosts chase Pac-Man more relentlessly. In order to accommodate the older software, many computers had a "Turbo Switch" which allowed you to slow down the computer. However as time went on, developers got better at writing games with proper clocks and users stopped wanting slower computers, so the Turbo feature disappeared.
Pressing this button injected new life into the sluggish Duke Nukem and the game took off, full of pixellated-sprite glory and witty one-liners. I recommended to my friend that he disconnect the Turbo switch to prevent any future accidents.