EDM is one of the most, but also one of the most complicated traits of Set 10. In short the trait grants you an item that allows you to select one of your four EDM units. Your strongest EDM unit of each champion (Maximum 4) will transform into the one you selected and cast the ability. The units will copy the base ability of the unit and take their star level into consideration, but they will all use their own items to enhance the ability.
It's pretty rare to open with EDM in Stage 2/3 and is more something you pivot into with Level 7 after finding an EDM Lux Headliner. You roll for Lux 3 Star, field one of each EDM unit and start pushing for Levels. In late game you can add powerful units like Illaoi, Yorick and Ziggs to help close out the Lobby.
Currently I would recommend using the EDM Selector on Zac and not on Lux. The bonus CC you receive from everyone transforming into Zac often has a higher impact to the fight outcome. It's very unlikely for Zed/Zac/Jax to have AP items that would enhance the Lux Laser if you use the EDM Selector on her.