It is executed with →↓↘+ATTACK while X is at full health and on the ground. Like the Hadouken, it is based on a Street Fighter attack, the Shoryuken. Shoryuken (昇龍拳 Shouryuuken, 'Rising Dragon Fist') is the technique learned in Mega Man X2. Light's capsule appears next to the pellet. Collect the Life Energy item and restart the level (people usually jump into the large bottomless pit to restart at the halfway point rather than the beginning), and repeat until Dr. Depending on what console the game is played on, instructions will differ from this point: As the platform bursts out of the tunnel and through the sky, dash-jump off of the platform and ascend the wall right on top of the gate to Armored Armadillo. Play through Armored Armadillo's stage until the last rolling platform. To obtain the Hadouken, obtain every Sub Tank, Life Up and Armor Parts. As the health of those enemies is above 32, they are killed with 2 hits, except for the Utuboros that takes 3 hits, and the Wolf Sigma who is immune to it. It is the most powerful attack from the game, causing 32 damage, being able to destroy most bosses and enemies in a single hit, with the exception of Mole Borer, Anglerge, Cruiziler, Utuboros, Thunder Slimer, RT-55J and Wolf Sigma.

While X's health gauge is full and he is on the ground, input the combination: ↓↘→+ATTACK. In fact, in some versions of the game, X shouts the name of the attack, just like the Street Fighter series. It is based on the Hadouken attack from the Street Fighter series, and is executed in the same way. The Hadouken (波動拳 'Surge Fist') is a secret move Capcom included in Mega Man X and Maverick Hunter X. I found an article here that will go into details with how to achieve The Hadouken and the Shoryuken.

Megaman x 2 shoryuken sprite sheet