CirTun is a magic Runeword created by combining a Cir Rune of Ritual and a Tun Rune of Invocation.
How to Create CirTun
You can create CirTun by adding the magic Cir Rune of Ritual and the magic Tun Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Cir - Cast the same non-Channeled Skill 3 times in a row. (50)
Tun - Invoke the Rogue's Stun Grenades, Stunning and dealing damage to enemies. (100)
Overflow
CirTun can not overflow because the amount of Offering you receive from the Cir Rune exactly fills the required Offering of the Tun Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.