Alternation
Alternation is a guideline that tells you to expect variety between corrective waves within an impulse. If Wave 2 is a sharp, deep correction like a zigzag, Wave 4 will likely be a shallow, sideways correction like a flat or triangle. The reverse also holds. This isn't a rule that must be obeyed. It's a tendency that shows up often enough to shape your expectations. You use it after Wave 2 completes to narrow down what Wave 4 will probably look like. If Wave 2 was fast and violent, dropping 61.8% in a few bars, you can plan for Wave 4 to grind sideways and test your patience instead. Alternation also applies to complexity: if one correction is simple, the other tends to be complex. Knowing this keeps you from forcing the same corrective pattern into both positions, which is one of the most common counting mistakes traders make.
Wave 2 of the S&P 500 drops sharply as a zigzag retracing 61.8% of Wave 1 in three weeks. When Wave 3 completes, you expect Wave 4 to be a sideways triangle or flat lasting several weeks, retracing only 38.2%. That contrast in depth, speed, and shape is alternation at work.