The metre gap applies to the forming of the line-out, i.e. before the line-out starts.
[TEXTAREA]19.8 FORMING A LINEOUT
(n) Metre gap. Each line of players must be half a metre on their side of the line of touch.[/TEXTAREA]
Once the line-out begins...
[TEXTAREA]19.9 BEGINNING AND ENDING A LINEOUT
(a) Line-out begins. The line-out begins when the ball leaves the hands of the player throwing it in.[/TEXTAREA]
... and as soon as the ball touches a player or the ground, the line of touch disappears and the offside line becomes the ball.
[TEXTAREA]19.13 OFFSIDE LINES AT THE LINEOUT
(b) Participating players. One offside line applies to the players taking part in the lineout
(usually some or all of the forwards, plus the scrum half and the player throwing in). Until
the ball is thrown in, and has touched a player or the ground, this offside line is the line of
touch. After that, the offside line is a line through the ball.[/TEXTAREA]
Players are allowed to jump across the line-out in order to compete for the ball, however, there are conditions...
[TEXTAREA]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.
[/TEXTAREA]
However, players who do are not usually penalised if they do not affect play and retreat immediately (ie. if the offence has no material effect on their opponents).
[TEXTAREA](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.[/TEXTAREA]