After a tank gains Elite status, that tank gains the ability to use the "Accelerate Crew Training" feature.
Additionally, any Premium Tank automatically comes equipped with Accelerated Crew Training.
Crews under this feature are awarded experience equal to 2x the amount of experience(XP) earned during the match. This means that your Crew will earn XP towards their Skills/Perks faster than normal. There is no penalty for this acceleration, and the tank still gains Tank XP and Free XP.