In order to use their psychic powers a psyker must draw deeply upon the power of the warp. This is a risky and very dangerous thing to do, for the warp is inhabited by hostile creatures that can attack an unwary psyker during the few moments that he draws upon the warp to use his powers.
To represent the dangers of the warp, if a psyker rolls a 2 or a 12 when he takes a Leadership test in order to use a psychic power, he is attacked! The psyker suffers 1 automatic hit with a Strength equal to the roll of a D6, no saves allowed – a warp creature has tried to fry the psyker's brain! Note that on a roll of a 2 the psychic power works, even though the psyker has been attacked or killed.