Also called peak shift. An effect that happens when reading from magnetic media. Essentially, boundary bits may be interpreted as part of adjacent data values. This means the value of the original element, and the value of the one next to it may change. Another way of putting it, would be to say the timing detected (and hence the data read) for a Flux Transition can change, caused by the influence of a neighbouring flux transition.
The IPF (Interchangeable Preservation Format) decoder (library) generates bit shifting into the decoded stream automatically at the most likely position it would occur if the data was being written to a real disk for absolutely faithful reproduction of the real media properties. This helps emulation of certain protections, without any kludges or any extra information required to distinguish between data meant for mastering and data meant for actual use (emulation).