These cards just sacrifice permanents, while some sacrifice payoffs are outlets as well
Part of what makes Elenda such a problematic creature is the fact that after damage is dealt to creatures and players, state-based actions are checked to determine which permanents should leave the battlefield
Its very easy to hit something with Bear Down, and in the case of this card even hitting a token of some kind is acceptable, because you dont use up an entire card to do it, since later in the game you get an efficient creature that is hard to block
It doesn't copy any information about the object the card was before it was put into your graveyard
Another infinite combo revolves around sacrificing Gravecrawler to Phyrexian Altar and pinging an opponent with Diregraf Captain to net you the mana needed to cast Gravecrawler from your graveyard again
The only problem is that the populate effect goes on the stack first if you cast a spell that makes a token, so its better to cast this after you already have some tokens