I've seen many players (on stream or in competitive game) manipulate roshan aggro from one melee hero to another. Or for example from a hero to sylla's sprit bear. How does that work. How do you force roshan changing target ? Any answer and more general tips on how roshan react would be great. Thanks
You need to be the closest hero to draw aggro. Melee heroes technically have a range of 128, so all else being equal Roshan will pick between them. The solution is to move the unit that you want to be focused even closer than that 128 range (m+click on Roshan or just right click precisely) and then continue attacking him. At this point, your ally who is currently under attack will be able to drop aggro by A+clicking on you or retreating and re-engaging.