The support tranches can be thought of as bodyguards for the PAC bondholders

“When the bullets fly…it is the bodyguards that get killed off first. The bodyguards are there to absorb the bullets. Once the bodyguards get killed off…the PAC tranches must fend for themselves: they are exposed to all the bullets.” These are the most exciting sentences in CFA Vol 5 T/G

Haha. They make learning fun.