It is a non-fermentable sugar, such as lactose, and usually potassium sorbate, in a clear syrup form. I make fruit wines, ciders, and apfelwein myself and started using it recently. It is handy for taking the dry edge off of a wine that is just a bit too dry or tart. I don't believe I'd use it for full-out backsweetening--for one thing that could get expensive, plus I think that might be too much sorbate. The recommended dosage for the bottle I got was about 2oz per gallon, and that seemed to hit it about right for my blueberry-citrus and key lime wines. They ended up right about where I liked them, not really sweet but kind of off-dry. It's a perfectly acceptable additive used by lots of winemakers, and I don't see why it wouldn't be just fine in a cider, although I can appreciate the ideal of trying to avoid adding anything extra to a craft cider.
Simple step-by-step for using wine conditioner