Ella Fitzgerald and Count Basie Orchestra Booty's Blues 1979 live concert

“Trading fours” (where players alternate taking four-measure solos) is a common practice in jazz improvisation, and frequently the occasion for musical competition between the players who are trading off.  When such one-upmanship is foregrounded, whether they are trading fours or not, the players are said to be “cutting heads.”  As Ella and Booty try to meet each other’s musical challenges, they are definitely having a party, and the audience loves it.  You can’t separate the party from the music; it’s all one experience! Make sure you check out them trading/her scatting from the 6:45 mark where she is up against a wicked trombonist.