Thanks for reaching out sir! Whereas a conventional FMJ leaves the lead in its core exposed to hot propellant gasses during ignition, the encapsulated FMJ's base is completely shielded by a small brass disc. The heat of ignition is thus unable to vaporize any of the lead in the encapsulated FMJ's core, which in turn promotes a cleaner barrel and suppressor as well as a healthier indoor shooting environment. In essence, the encapsulated FMJ achieves cleaner performance just like a total metal jacket (TMJ) – it just uses a brass disc to seal its core off from propellant gasses instead of a single, continuous jacket.
You can see a cross section of the encapsulated FMJ here:
https://i.ibb.co/jG8z9Cc/ibb.png