Cult of Tzeentch
Temporal Instability normally has rules only for combat, which are that each round you roll 1d10 and on a 10 you disappear for 1d10 rounds, other than the flavour text which states that the character disappear about 2 hours each day. This is house ruled to also be used in other stressful, scary or exciting moments; whenever the character fails a WP-test for fear, terror or insanity, or when the GM deems appropriate, the player rolls as he would in combat. Also, when happening in a non-stressed situation, you can usually feel it coming.
To observers the disappearance can take on any number of strange looks, anything from simply vanishing to dissolving into a horde of cockroaches that scatter as they hit the ground.