It's done via DXVA (2.0) if you query DX on a Maxwell card (or even Kepler) you should get something like VP9_VLD_Profile0: DXVA2/D3D11, SD / HD / FHD / 4K (You can either manually do it, or there is a tool called DXVA checker or tester or something along those lines).

LAV Filters (an Open Source implementation of DirectShow/DXVA filters) VP9 support works with Maxwell cards for sure, I have a 780ti somewhere I can check Kepler also, but IIRC it should work.

That said this would work for many video players that use DXVA or have support for external DS filters e.g. VLC/MPC-HC or proprietary players like Splash, but it won't work in a browser. Chrome for example doesn't allow for external filters iirc so you'll see a pretty big CPU jump. For some reason it seems fine on VP9 1080p videos on a Maxwell card CPU is 2-3% but at 4K it jumps to 70-80% I have a feeling that since Maxwell has partial support in the PV block for HEVC and VP9 they might be able to decode 1080p with it and then do full CPU decoding for 4K, I wouldn't expect CPU decoding for 1080p to be so resource efficient otherwise.

Just some more info this is the DXVA GPU accelerated decoder http://imgur.com/a/9MPsp http://imgur.com/a/q8SfN It basically decodes a VP9 stream with a shader and outputs it into an image format inluding NV12 which means you can use PureVideo for direct ouput.

This one can work with either a dedicated media player or with Edge, Chrome does it's own thing and I'm not entirely sure what they do and I have no clue about Safari/Opera/Firefox so if some one wants to fill in about them that would be nifty.

Whilst technically DXVA is a software decoder https://en.wikipedia.org/wiki/DirectX_Video_Acceleration It does offload the heavy lifting to the GPU via shaders, so can still some what classify it as "hardware decoding" since softare decoder/encoders are usually defined as CPU only.