"Listen very carefully!"
According to The Klingon Way, "The verb Qoy means hear, not just listen. One must actually perceive and understand that which is being said." I take this to mean that 'Ij refers to being ready to receive sound or more sound, while Qoy refers to receiving sound.
The difference between yIQoychu' and yIQoyqu' is probably the same as the difference between Listen very carefully! and Listen, really listen! That is, not much.