An Extella Maneuver is much like the Musou Gauge of Warriors infamy. Pressing the Circle button will execute a powerful spread attack that encompasses the immediate target in the direction you steer the Servant towards and will prompt you to repeatedly press Circle to expend more EM to cause more damage. You will need to do this as a part of the tutorial missions anyway, but get used to using this; it is a great way of immediately removing Plants once you've felled them back to the ground.