Contesting possession and causing poor quality ball for the attacking team from set piece gives the defensive line time to go forward.
Players in support when pressure is applied and tackles affected provide the opportunity to regain possession and turn defence into counter attack situations.
Regain possession via:
making effective tackles and turning the ball over
re-gathering the loose ball
tying up the ball when it is off the ground
recovering the ball after it has been kicked
commitment to tracking the ball at all times
Re-gathering the loose ball occurs when the ball is placed after a tackle or is hit loose in a tackle, and when errors occur in passing and receiving a pass, usually because of defensive pressure. In Sevens, offside Laws are far less effective because so much of the play is general play which allows for defensive pressure.
General play is play not initially based on a source of possession from which offside lines exist for the defending team. These sources of possession are the scrum, ruck and maul in which an offside line exists through the hindmost foot of players in the formation. Players who are not involved in the contest for possession are offside if they are in front of this line before the ball leaves the formation.
Much the same applies to lineouts; however, from lineouts the line is 10 metres from the centre line down the middle of the lineout. Once the ball leaves the lineout there are no offside lines for the defence.