An Angel Suppressing Sigil is used to give control of a vessel to the human the angel is inhabiting. Unlike angel banishing sigils, it need not be drawn in blood, though it needs blood for the spell to kick in. Once activated, the sigil will briefly weaken the possessing angel's control, giving control to the host. However, even the slightest alteration to the sigil will render the spell useless.
In Holy Terror, When Castiel tells Dean that the angel Ezekiel died during the fall, Dean realizes that the angel possessing Sam is an imposter and tasks Kevin with finding a spell that would "power down" the angel so that he would not be in control.