How does twin scroll turbocharger work?

Twin scroll turbine housing. The TST housing derives its name from the geometry of the exhaust gas inlet into the turbine. Two different-sized scrolls are generally used, a primary and a secondary. Typically, the primary is open for low-speed operation, and both for high-speed use. This creates the ability of the TST to be a small A/R housing at low speeds and a large A/R at higher speeds.

TST designs are of merit in that they offer a better combination of low-speed response and high-end power. It would be difficult to configure the unit to control boost by effectively varying A/R. A wastegate is therefore still necessary to control boost pressure. Simplicity of the twin scroll turbine housing is its big selling point.

[Source- "Maximum Boost" by Corky Bell]