Masturbating couples

A male and a female masturbating beside each other. Although it seems to sound somehow strange to couples who never tried, you can find quite many couple who perform this kind of sexual practice more or less often. How can it be exciting to masturbate while a potential sexual partner is lying right beside you? The extra portion of excitement comes not from your own masturbation, but from knowing and feeling and hearing and seeing your sexual partner being envolved in masturbation.