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 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)