It can involve one's hand, trying to bring the two together for pleasure, but isn't always a necessity in the act.
Still, today, frot is rarely used, and if so, generally only means "to chafe" or when one is dancing with another, "to gyrate."
How the gay community adopted this term is a mystery. Why they would choose such a disgusting sounding word to describe a sexual act is another mystery. If they wanted the act to be taken seriously, a more proper word should have been chosen, as "frot" is incorrect.
"Fricitation" and "frontism" are the correct terms for what the gay community think of as "frot." Genito-genital rubbing or GG rubbing is a term frequently used by primatologists to describe the act. "Frotting" simply means "chafing."
Dave: Would it be a sin to frot that girl over there subway doors?
Sean: I wouldn't. I saw her frotting four dudes at the club last night. She probably gets around.