They start at 80 and level to 1, basically.
You would still have to fight same-level or higher level enemies and do higher level content to advance down the levels as you would with a normal class, and you would get bumped to max level of an area (region, etc.) +1 once you had progressed below the level to prevent underleveling all upper-level content. WvW and PvP would still work the same as normal, with the class getting level set to 80 (or what have you). For gear they would start out in exotics and could stay in them throughout the de-leveling process.
And to even have one of these classes you would need to have atleast one max level of another class, so it would like a prestige class.
Potentially, I think you could use these classes as fast-tracks to getting legendaries for regular classes until they leveled beyond mat-harvesting range, since Legendary weapons are Soulbound on Use. They would also just be neat as a different way to play the game.