Snapping the cursor

https://twitter.com/pitiwazou/status/1230438511322550273 In this GIF demonstration pitiwazou is pressing the A key to create markers and then set the cursor at the midpoint. Can someone tell me how to set things up in blender to do this. When I press the A key everything in the scene gets selected. Thanks

  1. shift HOLD
  2. Right mouse down on a vertex
  3. shift RELEASE
  4. PRESS ‘A’
  5. move mouse to the other vertex
  6. RIght mouse RELEASE

Thank you jerzygorskiart. That clarified things for me and now it is working. I appreciate your description.