The exact nature of the protection offered by this layering remained a mystery for some time, but it was eventually revealed that Chobham armour works in a manner somewhat similar to reactive armor. When the armor is hit by a HEAT round the ceramic layer shatters under the impact point, forming a dust under high pressure. When the HEAT round "burns through" the outer layers of armor and reaches the ceramic, the dust comes flying back out the hole, slowing the jet of metal.
Modern tanks also have to face KE-penetrator rounds of various sorts, which the ceramic layer is not particularily effective against. For this reason many modern designs include additional layers of heavy metals to add more density to the overall armor package. The metal used appears to be either tungsten or, in the case of later M1 Abrams tanks, depleted uranium.
The effectiveness of Chobham armour was demonstrated in the first Gulf War, where no Coalition tank was destroyed by Iraqi ones. In some cases the tanks in question were subject to multiple point-blank hits by both KE-penetrators and HEAT rounds, but the lower power of the T-72 and T-64 guns left them completely incapable of penetrating the armor. To date only one Chobham protected tank has been defeated in combat, an M1 that was hit by an advanced dual-warhead HEAT wire guided missile in the second Gulf War.
Chobham armour is used on the Challenger II, the Leopard II and the M1 Abrams series of tanks.