Sentence:  The <WSD>crossback</WSD> elastic strap not only gives greater excursion both in scapular abduction and in arm flexion but it helps to prevent the control attachment strap from riding over the shoulder during extreme arm flexion, such as when the amputee is working in areas over his head. 
Meaning of the word marked with <WSD> tags:  Two pieces on the back of an item (for attaching or bracing it) which form an "X".