Your Visual Style Guide may recommend for you "an elongated silhouette"; your What Not To Wear may tell you "Avoid elongated lines." What does these look like, exactly?
Here's what they're not:
The lines in this pic all meet at least one of two criteria:
1. they're no longer than the section of the body to which they correspond,
2. they're no longer than what one would expect from a typical representation of the corresponding garment.
So, for example, the line at the torso isn't longer than the torso. That meets criterion number 1.
And for criterion number 2, the line at the boots is no longer than we expect a typical boot to be, the line at the blazer is no longer than we expect a typical blazer to be, and the line at the jeans is no longer than we expect typical jeans to be. (In fact, it's a bit shorter -- adding a Gamine element there.)
So the overall effect, in the mind of the viewer, is that no lines are longer than they "ought" to be.
Elongation, by contrast, looks like this:
Specifically, the sweater is both longer than the typical sweater, and longer than the torso; the shirt is longer than the torso (and a bit longer than the typical shirt); and the boot is much longer than the typical boot.
This is elongation.
The second criterion in each example above is more important in how a garment reads.
In other words, what the viewer expects from the typical garment is a key factor in determining whether a line reads as elongated.
For example, a baby doll dress does not usually read as elongation, because the line created is actually much shorter than what a viewer expects from a dress.