From BlenderWiki

Loop Subdivide

Description

Loop Subdivide (also known as Loop Cut…) splits a loop of faces by inserting a new edge loop intersecting the chosen edge. The tool is interactive and has two steps:

1. Pre-visualizing the cut

The cut to be made is marked with a magenta colored line as you move the mouse over the various edges. In (1. Pre-visualizing the cut), the mouse cursor was located where the white circle is located. This caused the loop line to appear at the mid point of the edge. The to-be-created edge loop stops at the poles where the existing face loop terminates.

2. Sliding the new edge loop

Once an edge is chosen via LMB , that edge is highlighted in green (2. Sliding the new edge loop), and you can move the mouse along the edge to determine where the new edge loop will be placed. This is identical to the Edge Slide tool. Clicking LMB again confirms and makes the cut at the pre-visualized location, or clicking MMB forces the cut to exactly 50%.

(3. Loop split edge completed) shows the new faces and edges, “A” and “B”. The view is rotated so that the new faces and edges are clearly visible from the top of the sphere.

1. Pre-visualising the cut.

2. Sliding the new edge loop.

3. Loop split edge completed.

Options

1. Previsualising the cut

• Upon initial activation of the tool 3D window header changes to show the Number of Cuts (Initial Loop Split header). Entering a number with the keyboard, scrolling Wheel or using + NumPad and - NumPad changes the number of cuts (maximum of 130). These cuts are uniformly distributed in the original face loop, and you will not be able to control their positions (no step “2”).
• S changes the cut to “smooth” mode. By default, new vertices for the new edge loop are placed exactly on the pre-existing edges. This keeps subdivided faces flat, but can distort geometry, particularly when using Subdivision Surfaces. If smooth mode is on then new vertices are not placed on the previous edge but shifted outwards by a given percentage, similar to the Subdivide Smooth command.

2. Sliding the new edge loop

See the Edge Slide tool description (which also contains some interesting examples…).