Wargaming (on RU portal) made a post, explaining what’s going on in the new 9.3 ricochet mechanics.
How it works now
A ricochet is a deflected shot, that hits the armor under a very steep angle. In World of Tanks case, if the AP or APCR (subcaliber) shell hits the armor under the angle of 70 degrees or greater, the shell will automatically ricochet – it will be deflected by the armor without doing any damage. For HEAT shells, the angle is 80 degrees. HE and HESH shells on the other hand do not ricochet at all. Ricochet check is performed when the shell hits the armor (normalization is performed after ricochet check).
Ricochet however does never happen when the “three calibers rule” is applied. This rule says that when the gun caliber (gun, not shell) is three times higher than the armor thickness, ricochets do not happen (HEAT shells do not use this rule however). Furthermore, ricochets do not happen when external modules of the tank are hit (tracks, optics and gun).
What is changed
Currently, two types of ricochets can happen. A shell can ricochet into a the tank (for example in a shot trap) – if, after it ricochets, the shell trajectory aims again at the tank, the shell tries to achieve penetration yet again. If it fails, it disappears. If the shell ricochets off the tank altogether in first place, it disappears.
In 9.3, after the first ricochet happens, the shell will continue on new trajectory in any case (it does not disappear regardless of where it flies). When that happens, APCR and AP shells lose 25 percent from their base penetration value, HEAT shells keep the same penetration value. If the ricocheted shells (with reduced penetration) hits another vehicle, the hit will be calculated based on normal game mechanics. Second such ricochet is not possible (if the shell ricochets again, it disappears).
Ricochets of your shells, hitting an enemy and ricocheting off, damaging your teammate, will not be punished.