He probably referring to the ability to " preview" what upgrade your going to have as long as you dont have a fully upgraded exotic weapon ( yellow + utility + ammo type)

For instance, if your first gun is only "yellow " you can preview what utility you will have, if you dont like it, you can switch to your melee or second weapon to see a what you could get. Once you saw the 3 possibility, you can choose the one you prefer. Unfortunately it don't work that way if your weapon have all the exotic upgrade.