Here is an aftermarket quick-spool valve used on a Cummins engine. The actuator for the valve is off to the bottom left of the picture.
The Cummins is an inline-6 diesel with a long log manifold. Due to its length, the manifold comes in three pieces with slip joints to allow for thermal expansion and contraction to prevent cracking. Between the exhaust manifold and turbine housing is the aftermarket quick-spool valve with the actuator hanging off the side.
Why the quick-spool valve? Well, there’s three turbos on this engine… It’s a compound setup with a single high pressure turbo feeding twin low pressure turbos. The single high pressure turbo is the one attached to the exhaust manifold. The exhaust leaves the HP turbo and is split into each of the LP stage turbos. After going through the turbo LP stage turbos, the exhaust gases are merged back together into a single exhaust.
On the cold side of things, air is sucked into each of the two LP stage turbos where it gets compressed. Both LP turbos then shoot the air into the single HP turbo for more compression work to occur. Massive boost pressures can be can be achieved by compound setups because the second stage works as a multiplier. If the LP stage turbos run a pressure ratio of 3 and the HP stage a pressure ratio of 2, then the final pressure ratio at the intake manifold will be 6 (~73.5psi).