The Plane itself resembles a volcanic wasteland. The users enter the plane by gathering some dirt in their hand and compress it until it glows red. On the plane the user doesn't always take a form, Lexi was able to eavesdrop on a conversation between the Monk and Scorch who were not aware she was there. The Espheni have a physical device that can be used to connect to the Plane. While its overwhelming to regular humans, people who were formerly harnessed can use the device and understand the Espheni though it causes high body temperature after just a couple of minutes of usage. Prolonged use even with spikes causes a detrimental physical effect.
The Espheni have hubs that Daniel Weaver theorized acts as relays for communication on the Shadow Plane. Anyone can use these hubs, but only with spikes from a harness could the person using it truly understand what was being stated.
The Overlords have used the Shadow Plane for an unknown amount of time to communicate wherever they are and in any situation to provide support. When an Overlord was captured by the 2nd Massachusetts Militia Regiment, it was planning to use the Shadow Plane to communicate with others and possibly send backup, however was murdered by Anthony.(Hatchlings)
A Shadow Plane hub was discovered by Marty, which Daniel Weaver theorizes is a relay station of sorts. When Weaver touches the Shadow Plane hub, he connects into the Shadow Plane which reveals many Espheni Overlords communicating with each other. It also gives the 2nd Mass and edge in the war to intercept their communications. (Respite) However, whenever someone touches the hub, they are overwhelmed by visions of Espheni communications and can't understand what is being said. (Everybody Has Their Reasons) Ben Mason is later able to better connect due to his spikes and discovers the existence of the Espheni Queen. (Stalag 14th Virginia) Following this, Ben connects repeatedly with the Shadow Plane and then cools down in an ice bath before writing down what he has learned on a chalkboard. He is able to avoid detection from the Espheni when using it and learn valuable intelligence. Despite his father's protests due to the impact on his health, Ben later secretly connects and discovers that the Alexis Glass-Mason that has returned is actually an Espheni clone intended to get close to Tom and assassinate him. Using what Ben discovered from the Shadow Plane, they are able to figure out that the Espheni Queen is at the Lincoln Memorial in Washington, D.C.. With no more use for the device and not wanting it to fall into enemy hands, Colonel Weaver then destroys it as the 2nd Mass and 14th Virginia prepare to leave their base. (Reunion) However, the use of the Shadow Plane draws a massive swarm of Black Hornets to the base. The swarm attacks, but is quickly driven off, though they kill Marty. (Reborn)