The player must go to any set of long stairs, then face the stairs and turn around, so Mario is facing away from the stairs. Now, the player performs a long jump. As soon as Mario jumps, they must hold backwards on the control stick. After they land, the player should rapidly tap the button and after a while, Mario should start zooming backwards at very high speeds, usually going through walls and doors. This is most famously used to get to the top of the Endless Stairs in Princess Peach's Castle. Normally, There is a trigger at the top of the stairs. This teleports Mario back to the bottom of the stairs. This prevents him from accessing the door without the required 70 Power Stars. However when the Backwards Long Jump is performed, Mario can build up a lot of backwards velocity, and the game does not teleport Mario properly because he clipped through the trigger, causing the game to not teleport Mario. You must have about 400 speed to do this. This is also the way Mario can go through walls. This glitch can do a lot of things. It can make the player complete the game with no stars and can access the "Black Room of Death".
This glitch also works with elevators and similar platforms. The player executes the glitch in the same way, but if they stop tapping while the wall is still behind Mario, he starts to run backwards very fast as soon as the player holds the control stick in a different direction. If the player doesn't stop tapping and has reached the top, Mario will teleport somewhere behind him (depending on what speed he gained while jumping onto the platform). This glitch was fixed in the Japanese Shindou edition re-release, as Mario's speed is capped when moving backwards. The glitch was also fixed in the DS remake as if when Mario attempts a backwards long jump, the camera will go behind Mario, making him do a normal long jump. This glitch is patched in the out in the Super Mario 3D All Stars Edition due to it being based off the Shindou edition. Thus most speedrunners use an American Nintendo 64 to speedrun, as that is the only version where BLJs are possible.