Try/Penalty Try

The referee will clearly raise one arm straight above his head.

His back will be towards the dead ball line.

Free-kick

The referee will raise his arm, bending it square at the elbow.

The arm will be pointing towards the team who have been given the free-kick.

 Penalty

The referee will clearly raise his arm in the air, pointing towards the side who have been given the penalty.

Obstruction

The referee will cross both his arms across his chest, like a pair of open scissors. This shows to all the players that one player has been stopped by another illegally 

 Forward pass

The referee will move his hands in front of his body.

Then he will pretend to pass an imaginary ball forwards.

He will give the scrum put in to the team that did not make the mistake.

 Knock on

The referee will raise his arm at head level, with the palm of the hand open.

Then, he will tap the palm of that hand with the other, to show to all the players that the ball has been knocked forward.

High tackle

The referee will hold is arm straight over his neck.

This shows to all the players that someone has made an illegal high tackle.

 Stamping

The referee will raise one leg, bending it at the knee.

He will then move it up and down, pretending to stamp on an imaginary ball.

The referee will bring both arms close into his chest as if he is holding an imaginary ball.

This is to show that the ball has not been released when the player in possession was tackled and did not release the ball.

He will give a penalty to the non-offending side at the place where the offence took place.

Handling in a ruck or scrum

The referee will bend forwards and lower his arm towards the ground.

He will then move his arm back and forwards, as if he has handled an imaginary ball on the gorund.