It's a reference to the end of Laputa: Castle in the Sky. In brief(-ish): Muska needs the heroine's pendant, actually some kind of control gizmo, to use Laputa's weapon systems (pictured in panel 6). Rather than allow this to happen, the kids decide to order Laputa to self-destruct by joining hands and saying the password "balus". The pendant releases its energy in a flash, blinding Muska and causing him to stagger backwards off the edge of the castle to his death.
It's actually rather dirty pool, them using it on him like that, but since Muska's a jerk, nobody likes him so nobody cares so it's okay!
Mamma AiutoBar-shu. (He's ordering the house beer (Bar-shu/balushu), but...) Come on!Balus!Ghibli AcademyMy eyyyyes!!Now I'll show you!Laputa's Lightning!My eyyyyes!!Burst through!Recruiting MembersYou should do it!Recruiting!The Dola ClanTotoro ClubRecruiting membersWe'll give you acorns.BonusGo explode...
