=sorry for the late reply=
well i call this ‘flinching’, and that is when your ranger (or any other classes) gets hit at the start of using their buster. when they get hit the class ‘flinches’ therefore not doing anything. although there are some classes that the busters work even if they have been flinched. An example is vampire hunter (vh), when she flinches her garlic bombs (buster) still drop… also pyromancer (pyro) when flinched his/its buster will still work, but without animation and the surrounding enemies just die .