Leaf shutters work by a series of blades separating to create a circular hole, that increases in size as the shutter opens - i.e. the opening increases to the maximum diameter during the first part of the exposure, remains open for most of the exposure time, and then closes down again.

If you are using a wide aperture, the time during which the shutter isn't fully open doesn't matter too much, as it is equivalent to exposure using a small f stop. When the lens is closed right down, the effect is that the shutter is fully open for much longer, simply because the hole allowes the full diaphragm opening to be uncovered.

Focal plane shutters are variable width slits in a roller blind, and expose the frame piecemeal - hence the distortion that can occur with moving subjects. The matter is explained better in various books, usually under "shutter efficiency".