The actual rotation angle can be set and retrieved by using rotationX, rotationY or rotationZ. This means that object3D.rotationY = 90 sets the rotationY to 90. But if we now use yaw(1) on that object the rotationY is not set to 1 but 91. yaw rotates the object along the local y axis and adds whatever value is passed. Often in a render loop you will see yaw(1) to animate a rotation this is similar to rotationY += 1.
I hope this helps a bit! Cheers, Jerome.
