In reply to purplemonkeyelephant:
When you say you "normally" lower with two hands on the dead rope, you're comparing it to what you know, which by the sound of it is a conventional tube device (ATC, etc). Assisted braking devices are in a different category: reducing friction in order to perform a lower always requires the use of one hand, whether pulling on a handle (as with a Gri Gri), rotating or squeezing the device. If the rope starts to move too fast, you simply do less of that movement.