During Live Investigation and Task sessions (both teacher-led), I often use a whole group discussion format just like a traditional classroom. Blended or not, there is no substitute for discussion.
Formal transitions happen twice during a double-block period. My students have to move from one section of my extra-large classroom (3 classrooms merged into one) and move to another section for a new lesson with a new teacher. My students have two minutes to transition from session to session, and we play a variety of interesting music on the surround sound stereo system to keep it fun.
My colleagues and I use a variety of hand signals in our classroom to avoid unnecessary disruptions and maintain focus and time on task. Three common hand signals: a signal to use bathroom; a signal for needing scrap paper; and a signal for asking a presenter to speak louder. We introduce all of the hand signals to students at the beginning of the year in a community-wide Town Hall Meeting.