you have to serve half of your enlistement then you can "Palace Chase".
after half of your initial enlistement period you can palace chase into reserves. once there you have to resign for another half of your initial enlistement period (additionally). So if you signed for 6 and PC at 3 years you have to do another 3 years as a reservist along with your 3 remaining years (6 years).
its kinda complicated to explain... even though im finishing my 4 years anyways then going reserves