Stephen Jones asks the following question: 'What are the laws regarding teams coming through the lineout after the ball has been tap backwards towards the scrumhalf?'
A player participating in a lineout is subject to off side, and LIABLE for penalty if they cross the line of touch or transgress the 15 metre line prior to the lineout ending. To answer the question below it is important to understand when a lineout has ended and thus when a player can come through the line of touch.
Law 19 refers in detail to the Lineout and to quote the law: The lineout ends when the ball or a player carrying it leaves the lineout.
This includes the following:
a) When the ball is thrown, knocked or kicked out of the lineout, the lineout ends.
b) When the ball or a player carrying the ball moves into the area between the 5-metre line and the touchline, the lineout ends.
c) When a lineout player hands the ball to a player who is peeling off, the lineout ends.
d) When the ball is thrown beyond the 15-metre line, or when a player takes or puts it beyond that line, the lineout ends.
e) When a ruck or maul develops in a lineout, and all the feet of all the players in the ruck or maul move beyond the line of touch, the lineout ends.
f) When the ball becomes unplayable in a lineout, the lineout ends.
Play restarts with a scrum.
It is also important to have an understanding of the off side law relating to the lineout.
Again to quote the law: LAW 19.14 OFFSIDE WHEN TAKING PART IN THE LINEOUT
(a) Before the ball has touched a player or the ground. A player must not overstep the line of touch. A player is offside, if, before the ball has touched a player or the ground, that player oversteps the line of touch, unless doing so while jumping for the ball. The player must jump from that player’s side of the line of touch.?Penalty: Penalty Kick on the 15-metre line
(b) If a player jumps and crosses the line of touch but fails to catch the ball, that player is not penalised provided that player gets back onside without delay.
(c) After the ball has touched a player or the ground. A player not carrying the ball is offside if, after the ball has touched a player or the ground, that player steps in front of the ball, unless tackling (or trying to tackle) an opponent. Any attempt to tackle must start from that player’s side of the ball.?Penalty: Penalty Kick on the 15-metre line
(d) The referee must penalise any player who, intentionally or not, moves into an offside position without trying to win possession or tackle an opponent.?Penalty: Penalty Kick on the 15-metre line
(e) No player of either team participating in the lineout may leave the lineout until it has ended. ?Penalty: Penalty Kick on the 15-metre line
Therefore in direct answer to your question a player in the lineout can tackle an oponent who is in posession of the ball once they land safely, however they cannot come through the line of touch to play a receiver until the ball has left the lineout / the lineout has ended. Should they find themselves there they must move immediately behind the off side line (in this instance the line of touch) without interfering with any oposition player.
Should the referee deem a player in this instance to be off side they would award a penalty on the line of touch at the 15 metre line.