Straight from my guide:
IBS (Invisible Bone Spirit): Ah the fear of all characters, what is it you ask? It is when after a certain distance, bone spirit becomes invisible! Similar to the invisible guided arrow glitch/bug. If you're losing life and you see nothing on screen, you are most likely getting hit by IBS or IGA (Invisible Guided Arrow). If you are getting hit and you see nothing, then you are most likely a victim of IBS or IGA. How do you do this you might ask? Read on about Bone Spirit.
Bone Spirit Information: Bone Spirit tracks yes. But what many do not know is that it tracks after it passes its targeted spot, or if you name lock someone. What I mean by by, after it passes its targeted spot? It is where you click your arrow, your BS will travel to, if it doesnt hit anything, it will begin to seek a target on its own. This is where IBS comes in. After traveling awhile after its targeted point, BS will turn invisible and still track, becoming the infamous IBS. How far it has to travel I have not yet found out, I just know that after awhile it turns invisible. Again, BS lasts 6 seconds tracking/seeking a target before it disappears. Also, Bone Spirit turns invisible if your opponent is OFF of your minimap, which means that their little red X with their name above their head must NOT appear on your mini-map for BS to turn into IBS.
My guide has pictures if u dont understand