A motivational shout, that the carriers of the Kakiyama use when starting a race, lifting the Kakiyama or when something else needs more power than usual.
In this region of Kyushu students often use “Yaa!” as a motivational shout during PE class. It is believed to have comee from a TV show and then spred throughout the region.
Another theory is that “Yaa” comes from the German “Ja”, which means Yes. It could have come to Japan during the second World War from German soldiers, when saying “Yes, Sir” in German.