Once you've beaten all eight Maverick bosses in "Mega Man X," the game doesn't have to end. There are still more secret items and upgrades to find, including the Hadouken, a powerful energy attack based on the iconic fireball thrown by Ryu and Ken in the "Street Fighter" series. To get the Hadouken, first you must have beaten all eight Mavericks and collected all eight Heart Tanks and Sub Tanks -- although you can collect the last Heart Tank and Sub Tank at the same time as you get the Hadouken. You'll also need all four of the Dr. Light Upgrades.
Select the Armored Armadillo stage. Make your way through the stage, avoiding or defeating enemies as normal.
Collect the Heart and Sub Tanks located in this stage if you haven't already. The Sub Tank is in a side passage not far from the beginning of the stage. When you drop down to the lower level, a Mole Borer will burrow out from this passage. Avoid it and run to the left to collect the Sub Tank. The Heart Tank is located down a pit to the right of the stage's second mine cart. Jump down the hole and head to the right, using Flame Wave to destroy the Mole Borer here. The Heart Tank is on a ledge above the passage.
Proceed to the end of the stage. However, instead of going to the exit, jump up and to the right from the mine cart as you near the exit. Wall jump up to a ledge where you'll see an energy capsule.
Jump off the ledge so that X falls and dies. You'll need to repeat this process four times. Once you've stood on the ledge four times (counting any previous times you made it to the ledge while playing the level earlier), the next time you climb up there, a Dr. Light capsule will appear. Note that the image of Dr. Light that appears is wearing a sleeveless karate gi, in homage to Ryu's costume in the "Street Fighter" series.
Jump into the capsule to receive the Hadouken.
The command for the Hadouken is the same as in "Street Fighter 2": Press down on the D-pad, followed by pressing down and to the right, then to the right, all in on continuous, rolling motion without interruption. At the end of the motion, before releasing the D-pad, press the attack button ("Y" on the SNES, "Square" on the PS2 or "B" on the GameCube). This motion is known as "quarter circle forward" among "Street Fighter" fans, which may help visualize it. As X fires, he will shout "Hadouken!" in a high voice. The quarter-circle may also be done to the left -- do it to whichever side your enemy is on.
You'll need to be standing on the ground to use the Hadouken, and you must have full health. Although it's tricky to use, this attack is devastating, capable of destroying most enemies in one hit.
Make sure you have plenty of lives before beginning the stage. If you need more, you can farm them by repeatedly killing the fat bat at the outset of the stage.
If you're playing a later re-release of the game and not the original SNES version, X may not shout "Hadouken!" when he fires.