What is the other technique creating a silhoutte from the source image using photoshop or illustrator?

Tracing the outline of the subject and cut it out on its own layer then blackening it.

Silhouette illustration is a trend in today’s design community that has become very popular. This form of illustration is focused on using and extracting the most basic forms of an object and turning them into art. It can be seen everywhere from Web graphics, Flash movies, and magazine ads, to full-blown TV ads. Although simple, silhouette illustration can reduce an object to its most simplistic form and still convey an emotion, style, and energy with little actual color and definition. In this tutorial I'll show how Photoshop can be used to create this type of artwork. You'll complete an exercise that will help build your portfolio, inspire you to add a new style to your design portfolio and push you to use Photoshop in ways that you may not have thought of before.

What is a Silhouette?  There are many definitions for the term silhouette. However, most have two core concepts in common. First, a silhouette generally entails the drawing of an outline of an object. Second, the background is typically removed from this drawing leaving only the object in focus. In short, a silhouette drawing includes only the images of the most visually important curves on the surface of an object, person, or structure.

Using the Auto-trace tool on the illustrator after placing the image and cropping it on photoshop ( or any other photo editing program) after the image is traced, convert it to one color

Its very simple. Trace the outline and fill that outline with black color without any background.