EX cancelling could mean 2 things. Most attacks you can cancel just by doing an EX and the delay will be cut off the first attack. Most attacks can be cancelled some way, but a lot of them can only cancel into EX moves. It could also mean something specific to Akiha's 623C, which is that the attack can be stopped midway and be cancelled into other moves unlike almost any other EX move. You don't have to let all the hits connect to go on doing whatever during a Akiha 623C.