The first iteration of WinTAP was a proof of concept that was essentially a .NET wrapper around WinPcap. This worked well, but performance was not as well as could be hoped for. The next iteration took the proof of concept, and instead of using WinPcap, implemented WinTAP as a kernel mode driver, using NDIS 6.0. This allowed for much better performance and stability.
From the official description:
WinTap is a packet sniffer and soft-tap developed to mirror packets flowing through an Ethernet interface. It is purely based on the NDIS which allows us to sniff packets and rewrite them to a second interface acting as a soft-tap. WinTap consists of two components,
1. NDIS 6.0 based protocol driver.
2. User mode soft-tap.
These two act in tandem to create a soft-tap, where the protocol driver sniffs the traffic and delivers to the user mode application. User mode application does the redirection logic and returns the packets to be rewritten to second interface.
WinTAP is licensed under GPL 2; source can be found on Github.