GREGOR Townsend, the Glasgow coach, has called for rugby's ruling body to clarify the law on what happens when one player jumps for the ball towards another player on the ground waiting for it.

As he waits to hear the fate of Finn Russell, the fly half he has been coaching for the last couple of years, he says the current ruling is a "grey area" where coaches and players really do not know what is allowed.

His complaint is that when Russell and Dan Biggar, the Wales fly half, collided in Sunday's RBS Six Nations Championship match, the Scot's big problem was that he was watching the ball and had no time to react when the Welsh player jumped into his field of view. If the ban stands, it fundamentally changes the principle hammered into young players that you keep you eye on the ball and nothing but the ball until you have caught it.

"It just needs a clear distinction on what you can and cannot do," Townsend said. "It is a watershed moment in the law. As a coach we have to make sure we are coaching according to the laws, so it will be interesting to see what happens on the appeal because there is going to have to be a clear statement on what the law is and what you are allowed to do when kicks are in the air.

"It is obviously a hot topic, there are two countries, Wales and Scotland, who are involved in it and commentators on the game who are making their thoughts known."

As he pointed out, it is a comparatively recent problem as tactics and skills have grown and coaches look to more and more different sports to see where they can glean an advantage.

"The game has evolved," said Townsend. "When I played and when I started coaching you did not see too many people to up to win the ball in the air. When you did, it was more that you knew you could fall awkwardly, it was a tough skill. The Australians were very good at it because a lot came from an Aussie Rules background.

"In recent seasons, maybe just the last season or two, collisions in the air have become deemed to be a bad offence. If there is intent, it should be something dealt with severely but now we are in this grey area of who is the receiver and what rights does a person have when the ball is in the air - whether looking just at the ball is insufficient as was the case last weekend. It is an important time for rugby to make sure we all understand the laws when the ball is in the air."

Coincidentally, Townsend's side, Glasgow Warriors, take on the Ospreys, Biggar's club, at Scotstoun this evening in a match that is bound to have a significant bearing on the end of season standings. Currently, Glasgow are top with the Welsh side third, four points behind, but it is a desperately tight battle at the top with five teams fighting for four play-off places.

To compound Townsend's problems, while the Ospreys have been allowed to play all their players from the fringes of the Wales squad, strengthening the side from the one that lost to Edinburgh last week, Glasgow have not only had no players back from the Scotland set-up but have lost Tim Swinson, who was called up as a replacement for Richie Gray.

"It is a huge game, very important for both clubs," he said. "The Ospreys have been leading the league most of the season and are very consistent. They are normally very good during this period, this is normally the time they come on a strong run. They have excellent strength in depth and having lost last week will be targeting this game as one where they can bounce back.

"That is the nature of this part of the season, it tests the strength in depth of the squad. I am very happy with the team we are putting out, a lot of them played well last season in Zebre, this group have trained well and at the team run. I can't wait for it to play."

He has been forced into changes. Josh Strauss, the No8, proved his fitness when he came off the bench last week and is handed a starting spot, but Lee Jones, the wing, is injured, forcing a shuffle with Niko Matawalu moving out from scrum half to cover while Ali Price get his first home start in the number nine shirt.

Up front, Ryan Grant returns as one prop while Rossouw de Klerk holds up the other side of the scrum with Zander Fagerson getting his reward for his try-scoring performance for Scotland Under 20s with a place on the bench.