Storing instantaneous velocity

Hi, first post ever here. Thanks for the help in advance. My level of Unity skill is monkey.

I am hoping to understand this more so then getting a solution from the community. Simplified, when I do something like this...

 void Update() {
   lastVelocity = Rigidbody.velocity.z;
   if (Input.GetKeyDown(KeyCode.DownArrow)) {
     if (lastVelocity == 0) { // sphere is at rest, not + or -
       // code to do something, i.e. rotate the camera facing -z.
     }
   }
 }

Background: in another script in FixedUpdate when I GetAxis the down arrow moves the sphere -z.

The action on line 5 above never executes. If I print out the velocity when I press the GetKeyDown key, it is always negative something. That is to say, if I press play and the sphere is at rest the velocity is zero. I grab the lastVelocity (should be zero), and then I press DownArrow even if I code it to grab the lastVelocity only once it still shows negative something.

Because of this, when the sphere is at rest, I can't code to have the camera rotate if the sphere is at rest and DownArrow is pressed.

Going Forward: I will want to expand the action to if (lastVelocity >= 0 && camera.Rotation.z != -180) to move the camera only when sphere is at rest or moving +z and camera isn't already rotated.

Maybe I am just not understanding the flow and execution order?

Thanks.