How does Collision2D.relativeVelocity Work?

I am trying to to get a ball to have an extra bounce off certain platforms.

I think this code should work but the issue seams to be that the relativeVelocity is not what I expect it should be. Sometimes it gives me a vector that already seems to be reflected and sometimes it doesn't. It seems to be unpredictable. Has anybody else tried using Collision2D.relativeVelocity?

     var bounceConstant:float = 20;
     function OnCollisionEnter2D(coll:Collision2D)
     {
         var vel:Vector2 = coll.relativeVelocity;
         var normal:Vector2 = coll.contacts[0].normal;
         Debug.DrawRay(coll.contacts[0].point,vel,Color.red);
     
         var force:Vector2 = Vector2.Reflect(vel,normal);//-(2*Vector2.Dot(normal,vel)*normal - vel);
         Debug.DrawRay(coll.contacts[0].point,force*bounceConstant);
     
         coll.rigidbody.AddForce(vel*bounceConstant);
     }