In reply to Fraser:
(as already stated not relevant to the accident in question, but...)
The instructions are actually wonderfully vague on the subject of how much of the lowering control should be from the handle rather than pulling on the brake rope...
'8. Lowering a climber in a top-rope situation
The belayer gradually pulls on the release handle, without letting go of the brake side of the rope. The handle can assist in braking, but the rate of descent is controlled by the hand gripping the brake side of the rope. For better control of the descent, you can add friction with a braking carabiner such as the Petzl FREINO.'
For comparison, the standard caving descender is the Petzl Stop, which while being a completely different beast in how it works still has a (rubbish) handle and a brake rope. The standard advice there is that the handle should be _completely_ depressed and all control comes from the brake rope. All cavers seem agreed that if you feather the handle (as well as partly controlling the brake rope) you will definitely die, and failing that wear out the bobbins in super-quick time (cavers, unlike most climbers, actually wear out their descenders).
Personally I have decided this is silly and use it like a (rubbish) Grigri, but we will see if I do wear out the bobbins - or indeed die horribly
It is possible to use a Grigri and lower having depressed the handle fully, and I have even heard this advice given before. It may wear out the rope less quickly (which is not such an issue for climbers) but it is much harder work and it is probably silly, but the instructions do say that control of the speed of the lowering should be done from the hand on the brake rope...