HB wrote:The next image shows the E61 lever at the mid position. The two lower valves are closed and the upper valve is barely open.

More properly : The next image shows the E61 lever raised just above the mid position. The two lower valves are closed and the upper valve is barely open.

Not all e61s open the upper valve in the neutral mid position. Some have a neutral position where neither valve is open. This is shown on machines with a brew pressure gauge. After the shot, and the lever is fully lowered to release the brew pressure, the brew pressure gauge retains the brew pressure. This pressure is held until the lever is raised above the neutral mid position. If the upper valve opened in the neutral mid position, the pressure would drop as soon as the lever entered the neutral mid position.

With a little "work", you can set up nearly any plumbed in E61 machine to do really good, manual soft pre-infusion this way.
With some coffees, this seems to result in significantly improved mouthfeel and overall experience.