Sentence:  A struck joint is made by running the trowel along the joint before the mortar is hard, either pressing in at the top: upstruck (or weathered), which exposes the mortar to the weather, or pressing in at the bottom: <WSD>downstruck</WSD> (or struck). 
Meaning of the word marked with <WSD> tags:  To point (finish a joint) by pressing mortar in at the bottom.