What is WASAPI?

WASAPI (Windows Audio Session API) is Microsoft’s multi-channel audio interface for communication with audio devices. WASAPI was introduced with Windows Vista™ and is supported by Windows 7 and later versions. WASAPI delivers an unmodified audio-stream to a sound device, and provides similar benefits as ASIO.

The Windows Audio Session API is Microsoft's high-end method for interacting with audio devices. It was introduced with Windows Vista and is available in later versions of Windows.

How does WASAPI work?

WASAPI enables client applications, such as WATCHOUT, or any application using audio, to control the flow of audio between the application and audio playback devices. It allows delivery of an unmodified bit-stream to a sound device.

WASAPI provides exclusive access to the audio devices, bypassing the system mixer, default settings, and any effects provided by the audio driver. Unless your audio interface has an ASIO driver, WASAPI is the recommended Audio Output Mode for Windows. WASAPI replaces all Microsoft Windows legacy output modes including Kernel Streaming and Direct Sound.

Source: https://msdn.microsoft.com/en-us/library/windows/desktop/dd371455(v=vs.85).aspx