Air-start SRB
I know that some Atlas launches do this http://www.spaceflightnow.com/atlas/ac164/031128launchtimeline.html, but here they are replacing one set of burned-out srb's by another set to maintain thrust at a level that the rocket structure can withstand, and to delay some of the thrust until the vehicle is above peak atmospheric drag levels.
But in the Japanese case, a mere 10 seconds after launch seems very early. Maybe TB has some observations?