I just picked this 1921 S morgan up. Looks like trailing die wear, base of 2 is disconnected from top
The best way I can explain it it's a fancy wording for a worn die or die scratches, lines, etc. Looks to me it's just the appearance of the toning as it goes under the "9" but has a difference on the "2".
I was going to post something about 'trails' on zincoln cents, but I got distracted by the Morgan Trail in California.
1921's, especially San Francisco coins, are notorious for having the dies pushed well past the limit. This is typical San Francisco fare; the die is so old and worn that the peripheral letters and numerals are being "drawn" towards the edge, Bust Half-style. It's quite common with them.