Jbobo
Watch it in real time (the first few seconds) several times.
http://www.bbc.co.uk/sport/0/rugby-union/31494355
Russell is already in the position he more or less ends up before biggar is even in the air (who is running full pelt but he's still on the ground just as he enters frame), he then jumps to grab the ball from above Russell and lands on top of him just to the pace he's moving at. Russell also clearly has his eyes on the ball and doesn't see Biggar coming.
He does attempt to duck slightly but to get out of the way is almost impossible.
He's not really guilty of anything as he is there to catch the high ball, clumsy to not see the welsh player and compete for it better but I wouldn't call it intentional in the slightest. Honestly I'd question a yellow card.