So, when you cast FM on a friendly target he gains some critical, and his critical damage increase your critical rate. Now when you change talents to one without focus magic the buff u gave to a friend is not removed, so the Focus Magic is not removed and u still benefit from it from your friend. I posted a hotfix who was on retail.
Hotfix (2009-04-21): "Focus Magic and Hunger for Blood should no longer last through a talent switch.
So, when you cast FM on a friendly target he gains some critical, and his critical damage increase your critical rate. Now when you change talents to one without focus magic the buff u gave to a friend is not removed, so the Focus Magic is not removed and u still benefit from it from your friend. I posted a hotfix who was on retail.
Hotfix (2009-04-21): "Focus Magic and Hunger for Blood should no longer last through a talent switch.