Complex sentence: When a control rod is inserted deeper into the reactor it absorbs more neutrons than the material it displaces (often the moderator), resulting in fewer neutrons available to cause fission and reducing the reactor's power output.
Split into simpler sentences: When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces (often the moderator). This action results in fewer neutrons available to cause fission and reduces the reactor's power output.