Sentence:  York came to me first, whilst the groom stood at Ginger's head. He drew my head back and fixed the rein so tight that it was almost intolerable; then he went to Ginger, who was impatiently <WSD>jerking</WSD> her head up and down against the bit, as was her way now. 
Meaning of the word marked with <WSD> tags:  To make a sudden uncontrolled movement.