Sentence:  I do n't know whether he was after our rider , who had gone by a minute before , or whether he was simply scouting conditions ; but when he passed us by , a musket roared , and he reared his horse , swung it around , and <WSD>began</WSD> to whip it back in the direction from which he had come . 
Meaning of the word marked with <WSD> tags:  take the first step or steps in carrying out an action