This works because the RAM location that is updated in the code as Tennis is played happens to coincide with the place where level data is stored in Super Mario Bros. The amount of time you spend playing Tennis, as well as the match score when you remove the cartridge can all affect which of the 256 worlds you load into, and it's totally possible for you to end up in one of the standard 8 worlds by chance. Reset the console, hold A+Start in the title screen, and voila, you will load into a glitched world. Remove the cartridge whenever you are ready, then put the Mario cartridge back in. Once the game has loaded, hit the reset button on your Famicom, then play a rally or two of tennis. Carefully remove the cartridge from your system and replace it with Tennis. Load up World 1-1 of Super Mario Bro s and wait for a few seconds.
To do so, it will require the Famicom console, a Super Mario Bros cartridge (obviously) and a Tenniscartridge.
