The spell creates a magical impact shield which surrounds the target and absorbs all hits until it breaks. The shield can break either from excessive damage, or from the effects of time. The time duration and strength of the shield is dependent on the caster's skills.
As the shield takes damage, it will change colours to indicate how much of its current strength has been lost. It will move from invisible down through dull red, bright red, wobbling orange and to flickering yellow. Note that these do not indicate any absolute strength, only a percentage of the current strength of the shield, not the original strength. Just prior to the shield breaking completely, the hit that is responsible will actually break through in that particular spot. The rest of the shield will survive for a few moments, blocking the other attacks, until it completely unravels.
If the shield is at one of the lower levels and ceases to take damage, the shield will move back up the scale to invisible again at a constant rate. I.e. a shield that is twice as strong will take twice the time to regenerate. However, once it has regenerated up to invisible again, the new 'strength' of the shield will be weaker than it was before the damage was taken.
In addition to damage, the strength of the shield wanes over time. As time passes, the maximum strength of the shield will continue to fall, until the point at which it breaks completely, even if it has taken no damage whatsoever. This is also believed to take place at a constant rate, so that a weaker spell breaks much faster than a stronger one, even in the absence of damage.
A wise wizard will therefore keep an eye on the duration of their shield. If combat ensues while the shield is nearly at the breaking point, it is doubtful that it will last more than a hit or two before breaking, leaving the wizard practically defenceless.
The time which the spell lasts without taking any damage ranges from about 2 minutes for low skills to up to about an hour with high levels, though it of course varies considerably between spells. Arrant times also often appear such as thirty seconds, or in rare cases, many hours.
The spell is not powerful enough to be used as a primary means of defence in combat. It is broken relatively easy by any reasonable opponent, and is therefore used primarily as a backup to Endorphin's, both for the hits that pass through, and for after it falls. For those wizards who do not use Endorphin's, it serves as a backup for those hits not dodged or parried.
Since it does not matter which type of shield you use, it pays to cart around only the lightest possible shield. Currently (aside from one rare exception), the lightest available shield is the mounted pewter shield, available in the Djelibeybi market place for around 300 tooni.
Each handful of ash weighs 1/9th of a pound, and so it is relatively easy to carry large amounts. It is readily available with a broom from many smithies, such as those on Filligree and Elm Streets in Ankh-Morpork, or Market Street in Bes Pelargic. It may also be obtained in the remains of certain campfires, and, less safely perhaps, from the remnants of vampires.