How can it get an image without a detector or something on the other side of the object? Is it just capturing reflected back x-rays?

Yes, that's what backscatter means. (Incl. Reemitted) Usually this is more accurate but requires higher doses (not necessarily received) for good quality images, advanced processing and sensors. Used sometimes as backscatter CT or in scintigraphy (looking at people who have been given radioactive contrast).