The swivel (or umbrella holder) is designed in a way that the umbrella does not go exactly 90 degrees into the shaft. It is tilted upwards a bit – this is done since the flash mounted on the swivel is not 100% aligned with the center of the umbrella. If the insert was angled at 90 degrees, light from the flash would hit only the top of part of the umbrella, creating uneven light.
So the umbrella is tilted upwards a bit. However, the swivel is one and mounting options are many – you can use big or small umbrellas, place the umbrella in different positions within the shaft or shoot through only half open umbrella. Since the shaft if fixed, sometimes you’ll end up with the light going only to the top or bottom half of the umbrella – this is why you’d like to place the flash as close to the umbrella axis as possible.
