They were doing a special remix ofa couple of their more famous songs for the fans and were using a lot of dance moves that would make the fangirls (and boys) squeal and/or faint and/or possibly become wet (get hard ons). So of couse everytime Yongguk saw Zelo dance he would get more. Finally one day at practice the maknae made a "mistake" and Yongguk took advantage of it.