Instant return means that you press the shutter release, the mirror flips up, the shutter opens, the shutter closes, then the mirror flips down again. As far as I know, every SLR made over the past ~45 years has had an instant return mirror, starting with the Asahiflex IIb in 1953. Before that, the mirror would stay up until you advanced the film, which I'd imagine was decidedly inconvenient! I'm not entirely sure what you're specifically refering to in your second question. Do you understand the basics of how an SLR works?