I got to reading AD&D 1e lately and I see the DMG states something more than what the PHB states when it comes to Hide in Shadows. Seems to me you can move while Hiding in Shadows. We never played that way since 1978. Strange. TSR used to have an answers website for things like this. Where can we find clarifications of the rules these days? AD&D needed alot of that over the years I remember.
Ex: Party opens a door into a room with 2 Orcs. Melee insues. Theif starts with Hide in SHadows before he enters the room. The orcs have no idea the party even has a theif. Can the theif move in shadows around the edge of the room to get behind the orcs without being noticed and get a backstab.