When the first hand of a round is played, if the cards held in hand prior to play contain a straight, gains Mult equal to the rank of the lowest card in the straight.
(If multiple straights are possible it chooses the best straight available.)
For instance, if you have AKKQJT5432 in your hand and you play the pair of kings then you have two possible straights in your hand prior to play, AKQJT and A5432. This joker would choose T as the lowest rank in the straight in the first possibility and 2 as the lowest rank in the second. T is the larger of the two, thus this joker would gain +10 mult.