Chainsaw Beam is the combination of Wave Beam, Spazer, and Plasma Beam while Charge Beam is unequipped. It can open doors and destroy destructible blocks that the beam is normally capable of destroying.
The Chainsaw Beam has a very high rate of fire, but has a very short range (hence the name), and doesn't have any visible effect or projectile. Damage from the beam is only dealt during a Power Bomb explosion, with a maximum of seven hits per explosion. It has the same damage type as Super Missiles.
To equip Spazer and Plasma Beam at the same time, open the inventory screen, equip Spazer, and move the marker to any of the Boots items. Then, press Left, and press A immediately afterward. Plasma Beam should then become equipped alongside Spazer with "VAR" next to it.