The shield keys allow you to activate your manual shield. When it is active, the manual shield continuously depletes your reserve shield power. However, if a blast hits you when you are manually shielded, the impact causes no further loss of shield power. In contrast, if a blast hits you when you are unshielded, your automatic shield must surge to protect you and the loss of shield power is quite large.
Similarly, the shield depletion caused by a missile is much less when you have manually shielded yourself. Of course, if you activate the manual shield at the wrong time or for too long, you waste it needlessly.
