Throughout Bleach, fans follow Ichigo Kurosaki as he grows and learns about his latent supernatural abilities. Thanks to his father's former Shinigami rank and his mother's pure Quincy blood, Ichigo is able to use a wide array of Hollow, Quincy and Shinigami abilities. He's even able to combine these energies and create techniques that no other being would be able to use. Of these abilities, one of the most destructive and powerful has to be his Combined Gran Rey Cero-Getsuga Tensho technique.
As the name suggests, it is an attack that combines two separate abilities to create one large one. The first part of the attack is a technique used by the Espada that combines the blood of the user with a normal Cero attack, which is a concentrated blast of spiritual energy. The blood of the user acts as a catalyst that increases the Cero's speed and power, creating a stronger attack that can destroy large areas in a short amount of time. The attack also distorts the space around it as it compresses and decompresses through the air. It's so powerful that it was forbidden within Las Noches.
The second half of the attack is Ichigo's Getsuga Tensho attack. Passed down to him by his father Isshin, this Zanpakuto technique is a slash attack where the user concentrates their spiritual energy into the blade of their weapon and brings it down onto their opponent. Because of the amount of energy released, it is highly destructive, as seen when Isshin destroys a large chunk of the Fake Karakura Town when fighting Aizen.
When Ichigo uses the technique, he can choose to unleash it either using the smaller Zangetsu blade or the large one. When using the smaller blade, all he has to do is point it at his opponents and make a small slashing movement to release the attack. When using the larger sword, the attack takes on a black color. When used as a True Bankai, it is so powerful that, had The Almighty not reversed Yhwach's death, it would have killed him for good, even after he had absorbed the Soul King. Ichigo is also able to use the technique to augment the blades of his swords.
Ichigo can combine these two techniques in his Merged Hollow Form by using his blood to increase the speed and power of his Getsuga Tensho. This is the attack that knocked back Yhwach and proved to the Quincy King that Ichigo was powerful enough to be taken seriously. Even before combining it with a Gran Rey Cero, the Getsuga Tensho technique is highly destructive -- increasing its speed and attack power makes it almost unstoppable for an average opponent.
The only reason Yhwach was able to withstand the attack was that he had already absorbed the Soul King when Ichigo decided to combine these two techniques. Had Ichigo been fighting any other opponent, they most likely would have been obliterated by the attack, given how powerful it is.
