--- source/blender/blenlib/intern/math_rotation.c
+++ source/blender/blenlib/intern/math_rotation.c
@@ -78,7 +78,13 @@ void mul_qt_qtqt(float q[4], const float q1[4], const float q2[4])
 	q[2] = t2;
 }
 
-/* Assumes a unit quaternion */
+/* Shortcut for q v q* when v is actually a quaternion. This removes the need
+ * for converting a vector to a quaternion, calculating q's conjugate and
+ * converting back to a vector. It also happens to be faster (17+,24* vs
+ * 24+,32*). If q is not a unit quaternion, then v will be both rotated by
+ * the same amount as if q was a unit quaternion, and scaled by the square of
+ * the length of q.
+ */
 void mul_qt_v3(const float q[4], float v[3])
 {
 	float t0, t1, t2;