Sentence:  Interpreted in terms of <WSD>helicity</WSD>, this means that if the incident electron has positive <WSD>helicity</WSD>, then the outgoing electron has positive <WSD>helicity</WSD> for forward scattering, and negative <WSD>helicity</WSD> for backward scattering. 
Meaning of the word marked with <WSD> tags:  The quantized spin component of a moving particle along the direction of its motion.