Mirrored from Sudopedia, the Free Sudoku Reference Guide
The following image illustrates the concept:
This loop has 2 simple rules:
When we close the loop at node A, we can see that these rules are not obeyed by node A. It has two red links. Therefore, this is a discontinuous loop, with node A being the discontinuity. There can only be a single discontinuity in a discontinuous loop. A loop that violates the rules at multiple locations is not a valid loop.