Always Subbing is a rotation and field-position system. It is not primarily an attacking strike play.
The idea is to use the early touches in a set to deliberately move the ball toward the sub box side. The player who gets touched near the sideline can then leave the field immediately after the rollball, allowing a fresh player to enter without breaking the tempo of the attack.
The goal is not necessarily to find a gap on those early touches. The goal is to keep fresh legs cycling onto the field while the ball remains alive and moving forward. Done properly, the team maintains speed, pressure, and defensive discipline across the whole match.
The sub box side depends on the venue. The system should not be hard-coded left or right. The rule is: identify the sub box side, then use early touches to migrate the attack toward that side.