The armour plate (1, 2) is covered in pits (3), that have a special shape. Inside each pit, there is a charge with a detonator (5). Most bullets that hit the armour will hit inside the pit, as any object will obey the laws of least resistance and ricochet off the walls into the depth of the pit. The pits will have a shape slightly smaller than the bullet, so when the bullet hits, it will plug the pit.
The bullet (4) will hit the detonator with its tip, which will detonate the charge. The resulting gas expansion will slow down the bullet and not let it penetrate the armour.