Link Summons: Can You Really Do It with a Face-Down Monster?
A Direct Answer:
In a nutshell, no, you cannot link summon using a face-down monster as a Link Material.
As per the YuGiOh! official FAQ:
">A monster used as Link Material for a Link Summon must be face-up in the Monster Zone."
While some cards may have the potential to be face-up even when they are Link Materials, they explicitly must be facing up in the Monster Zone when used as a Link Material. To facilitate this, face-up Monsters are the staple Link Material option.
A Link Link Materials Discussion: If that first answer wasn’t straight and clear, then why?
Why the Focus on Face-Up Monsters Only:
In this setting where Link Summons rule supreme, the reason it all boils down to positioning a monster in a given turn. When summoning your Link Monsters, an opposing player’s Monster does get a chance to destroy its opponent’s card instantly.
Raigeki and Dark Hole, two examples for destructive forces, in Link Battles. When cards battle together, they clash due to their ATK or the results of their various forms that can lead to victories/losses and overall victory. The link Material also plays a key. Now, the actual situation involves the card battle positioning during the game in response the card battle to begin your Link Summon or have these cards destroyed with help using the cards as links on the field with link zones.