How does Xialla block blood outflow but not block blood inflow?

To understand how Xialla can block blood outflow without blocking blood inflow, it's important to recognize that inflow occurs through arteries deep within the penis, while outflow occurs through veins that are located very near the surface.

Xialla applies mild compression that is enough to compress the veins on the surface to slow the blood outflow. However the compression does not have to be so tight as to prevent the blood inflow through the arteries deep within the penis. 

In this way Xialla is able to slow the blood outflow to treat venous leak, without preventing blood inflow that allows the erection to build. 

Additionally, because Xialla is secured against the base of the penis, the ring is not pushed forward by the pressure of the inflowing blood. If inflow does collect behind the ring, a gentle pressure of the fingers to encourage the blood to flow under the ring is enough to start the erection. Once the erection starts, the ring is expanded so applies even less pressure on the arteries deep within the penis, and this allows the erection to continue to build.