A transient enhancement in the performance of JET discharges was first obtained in the L-mode regime when a peaked density profile formed by pellet injection was heated using Ion Cyclotron Resonance Heating (ICRH) alone. The enhancement of the central parameters observed in such modes was later combined with the edge related enhancement obtained in the H-mode regime and was achieved with combined ICRH and Neutral Beam Injection (NBI) heating. Since then Pellet Enhancement Performance (PEP) modes have been created in a number of discharges at various plasma conditions (Ip = 1 to 4.1MA; BT = 2.2 to 3.5T; heating methods varying from up to 16MW ICRH to 18MW NBI). Several PEP discharges have been analysed using the TRANSP code to try and establish the reason for the good performance and to assess the effects of the different heating schemes.