Dragon Fusions require a Mir Skill which is then followed by a Circle Skill. Evan's primary casting skill is Mana Burst, so to use a Circle Skill instead of Mana Burst would be a slight loss of damage. However, Mir can attack separately from Evan without the need of a cast animation. After a Mir Skill is used, Evan can input a Circle Skill WHILE attacking with Mana Burst, and Mir will conduct his corresponding fusion skill, effectively "cancelling" Evan's need to cast the Circle Skill.
Mana Burst has two-parts to its attack animation - first hit (MB_1) and second hit (MB_2) - with a very slight delay in between both animations. This is an additional layer to animation cancelling where the timing of casting Return! on the Circle Cancel during Mana Burst will cancel MB_2's animation but not its damage, and start the next Mana Burst cycle.
Credits: Ant (Youtube)