Quote ="Kosh"You can knock on after a tackle is complete and before playing the ball. It still counts as a knock on. Blaymire voluntarily released the ball after the tackle, making it a knock on. Alibert even awarded the scrum to Celtic.
It should have been play on and the try given.'"
Sorry,but youre wrong - below is from the rule book.
Losing possession 8.
– intentionally
– accidentally
A tackled player shall not intentionally part with the
ball other than by bringing it into play in the
prescribed manner. If, after being tackled, he
accidentally loses possession, a scrum shall be
formed except after the fifth play-the-ball.