Not long ago I've come across this interesting info that I've never seen mentioned in english speaking part of the Internet, so I've decided to translate and post it here.
Source is the "Manual of ammunition for 76mm field, tank and self-propelled artillery", this part is describing the functioning of BR-354P subcaliber shells:
There you go.The functioning of this shell is as follows: during the impact on the armor, the ballistic cap is destroyed and the core penetrates the armour creating a hole equal in size to the core's diameter. Penetration of armour is achieved by the use of kinetic energy of the core itself as well as that of the sheath until it impacts the armor.
When striking armour the sheath, made of mild steel, begins to break up and therefore the kinetic energy it passes to the core gradually diminishes until the base of the core has completely sunk into the armor. The amount of energy transferred until then is linked to the thickness of the armor as well as the angle of impact. As the thickness attacked diminishes as well as the striking angle the contribution of the sheath in penetration lessens and the other way around, as the thickness and impact angle grows so does it.
With the goal in mind of maximizing the contribution of the sheath in penetrating the armour, it's sides are designed to absorb the least amount of energy as they are destroyed and are just thick enough to obtain the required integrity of the shell when fired and the adequate centering of the core during impact.
The base of the sheath is instead designed to not be pierced by the rear of the core at impact, because only then it's energy can be fully exploited in defeating the armor.
This way the core, with the help of small diameter, high hardness and huge kinetic energy (both itself and that of the sheath), creates favorable conditions for the perforation of armor, which are: concentration of significant kinetic energy over a small area that creates huge stresses in the armour material and leads to it's failure.
After it passes through the armour, the core disintegrates and damages by fragments (both it's own and those of the armour) the insides of the tank. When striking the armour the subcaliber core heats up to a high temperature. This heats up the fragments, which when hitting the fuel create fires inside the tank.
The schematic of the action of subcaliber shell is shown on the drawing n.165.
The album with the relevant scans: https://imgur.com/a/F1tkvmE