Quaternion LookRotation issue

Hi! I am trying to simulate the movement of the left forearm with the positions of the joints of the left shoulder, left elbow and left wrist.

I am computing the difference vectors: leftShoulder-leftElbow and leftWrist-leftElbow. Then I make the cross product between both and I called it "x". Then I assume that the left forearm is always aligned with one vector so I assigned "y" to the vector leftWrist-leftElbow. Then I make again the cross product between "x" and "y" to obtain the orientation "z".

Now I have all the orientations that I want and I need the forearm to move like according to these. Therefore I used a Quaternion. Through some readings I try to use the function Quaternion.LookRotation with two inputs and it kinda works. I put the code below to better understanding.

 public class LeftForeArmReferencial : MonoBehaviour {
 
     public GameObject leftShoulder;
     public GameObject leftElbow;
     public GameObject leftWrist;
     
     private Transform leftShouldertrans;
     private Transform leftElbowtrans;
     private Transform leftWristtrans;
     
     private Vector3 leftShoulderLeftElbow;
     private Vector3 leftElbowLeftWrist;
     
     private Vector3 x;
     private Vector3 y;
     private Vector3 z;
     
     private Vector3 normx;
     private Vector3 normy;
     private Vector3 normz;
     
     private Transform trans;
     
     private Matrix4x4 m;
     
     void Start () 
     {
         trans = GetComponent<Transform> ();
     }
     
     
     void Update () 
     {
         leftShouldertrans = leftShoulder.transform;
         leftElbowtrans = leftElbow.transform;
         leftWristtrans = leftWrist.transform;
         
         leftShoulderLeftElbow = leftShouldertrans.position - leftElbowtrans.position;
         leftElbowLeftWrist = leftWristtrans.position - leftElbowtrans.position;
         
         x = Vector3.Cross (leftElbowLeftWrist, leftShoulderLeftElbow);
         y = leftElbowLeftWrist;
         
         normx = Vector3.Normalize (x);
         normy = Vector3.Normalize (y);
         
         z = Vector3.Cross (normx, normy);
         
         normz = Vector3.Normalize (z);
         
         m.SetColumn (0, normx);
         m.SetColumn (1, normy);
         m.SetColumn (2, normz);
         
         trans.rotation = QuaternionFromMatrix (m);
     }
     
     public static Quaternion QuaternionFromMatrix(Matrix4x4 m)
     { 
         return Quaternion.LookRotation(m.GetColumn(2), m.GetColumn(1));
     }
 }

My question is: how and why does this quaternion function work? I put the "z" as the first parameter and "y" as the second because they suggested it here http://docs.unity3d.com/ScriptReference/Quaternion.LookRotation.html but why is this way? Wouldn't it work if I changed?