This post seems to invite the old joke about the difference between theory and reality.
The way it's been working recently is that the lateral x-fer happens when you would have started training in the new bid. Meaning that if you were an ANC MD-11 waiting to x-fer to MEM, you would do so when your seniority would have had you training for MEM in the current training letter.
In the past, it has been when you would have activated instead of started training. Nevertheless, you will probably get to your new base faster if you are lateralling vice waiting to train.
It also seems that the x-fer dates are more firm than training dates, but I haven't watched them that closely.