By "rear curtain" I mean firing the flash towards the end of an otherwise normally metered and timed exposure - including in manual mode, in contrast to "slow sync" which at least on the X100 is timed automatically.
Preemptive - the fact that the X100 series doesn't have shutter curtains due to using a leaf shutter is completely irrelevant, "rear curtain" is just a name that stuck from the original design, with an electronic camera it should be possible to trigger the flash at an arbitrary point during the exposure.
