Some web-apps, like youtube, do the casting for video in a more complicated manner. Rather than actually doing the 'remote desktop in reverse' they essentially

Some web-apps, like youtube, do the casting for video in a more complicated manner. Rather than actually doing the 'remote desktop in reverse' they essentially instruct the Smart-TV to go to the same video end-point and start streaming. Thus, none of the rendering, decoding or even downloading will be done on your laptop. This is slightly worse from a control perspective, but quality wise it is great.