What is a winding check, you ask? Well, it's the ring you see on most fly rods just in front of the grip. It has two jobs: it protects the cork where it meets the rod shaft from chipping & it gives a visual transition from the grip to the rod. Some fly rods don't have winding checks, but the vast majority do (those that don't usually have a long thread wrap up the front of the grip).
There are many types & styles of winding checks. This variety allows us, when we're designing your rod, to have more options. We can make a bold statement with the check, or we can be subtle - such as matching the knurling on the reel seat hardware with some knurling on the check. It's stuff like that that can really make a rod nice.
However, one option some choose for six-sided bamboo rods is to have the winding check match the shape of the rod shaft. Hence, the six-sided winding check on bamboo rods is also an option.