Sentence:  The striking-platform features and the kind of <WSD>percussor</WSD> used are critical variables in determining the bulbar characteristics on flakes; the variable that is most critical depends on the hardness of the <WSD>percussor</WSD>, the angle of the striking platform, the amount of force applied, and the relative mass of both core and <WSD>percussor</WSD>. 
Meaning of the word marked with <WSD> tags:  A hammerstone or similar tool used for smashing, or chipping.