Sentence:  The primary function of a souvenir fetish is so that the offender can relive the actual event within the context of the fantasy, usually while <WSD>masturbating</WSD> to maintain sexual arousal and achieve orgasm (Canter et al., 2004). 
Meaning of the word marked with <WSD> tags:  To stimulate oneself sexually, especially by use of one's hand or a sex toy made for this purpose, often to the point of ejaculation.