Least Squares Moving Average: the endpoint of a least-squares line fitted over the last length bars, so unlike an SMA it has no lag against a straight trend: fit a line to a line and you get the line back.

offset steps back down that same fitted line rather than re-fitting, which is why it can shift the plot without changing its shape. See the x-axis convention on linreg in ./calc: x is 0 at the oldest bar of the window.