The action of flexing the knee to 90 degrees occurs in the front leg of Virabhadrasana 1 and 2 and in Utthita parsvakonasana.
This 1-minute, 19-second video shows correct alignment in bending the knee of the front leg in Warrior 1 and 2 and in Extended side angle pose. After purchasing the video, click on the YouTube icon to view it on YouTube. Save the URL in the address bar for permanent access to the video. Although the video may be viewed on a mobile device, the URL can only be found on a laptop or other personal computer.