https://www.youtube.com/watch?v=T3OhV7aP2mw
The manoeuvre begins at about one minute in. The Franks go from a line facing the enemy to a double vertical column, and as they close with the enemy, the two columns split apart to form crescents convexing toward the enemy line.
Kingdom of Heaven is one of my favourite movies, and I wish I understood this scene better.
Haha I came across this question by coincidence while googling about war tactics in the movie myself. Anyway, if you still need an answer, this is called a Frankish Charge and was an attack move with heavily amored cavalry to break down the middle of an army. The Muslims however developed the counter strategy shown in the scene.