Brilliant finishing tactic [zwischenzug]

Black is pretty much in despair since white is winning with an overwhelming advantage both in material and position. There is however a very nice finishing blow, using a neat little tactic. Hint is in the title

What is it in-between, though? 

Then Bf5 and Black loses a piece instead of much more. Still absolutely crushing but...

1. Bg6 Bf5 2. Bxf5 Rxd8 3. Bxh7

Not a zwischenzug - there is no capture, and hence no re-capture to sneak moves in between.  Also 1. Bd5+ wins the a8 rook directly, but the solution shown is best because after the black king moves an 3. Rxh8, the remaining black rook and bishop are both still en prise, and there's no way to save them both.

Correctly analyzed. But I still want to ask why it wouldn't be a zwischenzug. Zwischenzug means "in between" meaning a position where you prolong an obvious move by playing some other move before it. Bd5+ is an obvious move, but you can make it better by playing an "inbetween move" aka a zwischenzug.

A zwischenzug (German for "in between move"), by definition (at least the definition I've been taught), is a delayed response to an opponent's threats or tactics.  The tactics shown in this puzzle are a combination of two simpler tactics, 1. Bg8 is a deflection followed by 2. Bd5+ which is a revealed attack.

A true zwischenzug would involve the defender (black) either taking a piece (even a pawn), expecting an immediate recapture, or making a compound threat (such as a fork or skewer), but instead being met with a secondary threat which must be responded to immediately, and which fundamentally changes the material and/or positional result of the original tactic.  Since black doesn't make a tactical threat of any kind, there's no zwischenzug.

superb

superb

I like it. Got it on my second try. (I also thought Bg6 at first...)