A tamasic sacrifice is missing everything that matters. No proper procedure. No food offered to others. No mantras chanted. No dakshina given. And worst of all — no faith behind it. Such a ritual, Krishna says, is called tamasic.
Every essential element is absent. There is no method, no devotion, no generosity, no surrender. It is an empty shell of ritual — done mechanically, done because someone said to do it, or done out of obligation with zero feeling.
Among all the deficiencies Krishna lists, the most critical is "shraddha-virahitam" — the absence of faith. Without faith, no amount of correct procedure can redeem a sacrifice. Faith is the root; everything else is a branch.