1) Messi drives towards a defender, fixing him to his position.
While travelling he has heavier touches to maximise speed and eliminate recovering defenders. Messi drives at the defender at full speed, this allows him to go inside our outside. Ideally he would like to go left on his preferred foot, but the defender can also decide for him once off balance.
2) Forward runs from team mates distract other defenders, creating confusion and allowing Messi to drive into space.
3) As he approaches the defender he keeps the ball closer to his foot, so he can move it instantaneously and change direction.
4) The finish has clean contact, Messi tends to hit the ball with pace using the inside of the foot curling out to in.