Tempting though it is to read an emergency security fix into the 'critical' nature (something I haven't seen for years in the Windows Phone world), especially with the several high profile Internet plumbing bugs found in recent months, Microsoft says that the update is to 'test the update systems'. Which is fair enough and I doubt that the early adopters and developers on the DP programme will mind being treated as guinea pigs, especially as they get a new build of the OS with - no doubt - miscellaneous fixes and tweaks under the hood.
The update takes any Windows Phone 8.1 Update 1 device (that has been enrolled in the DP programme) to OS version 8.10.14192.280 - the 'firmware' version number of any device remains intact, so there are no changes to fundamental hardware parameters, it's purely a Windows Phone thing.
The previous OS version number for those on the programme was 8.10.14176.243 - and no, there's no changelog of any kind. Still, better update anyway, at your convenience, before the 72 hour limit is hit when you're in the middle of an intense couple of hours of work!
The idea that this is simply a 'test' of the critical update mechanism is validated by no updates being seen on any of our other Windows Phone devices, the ones that aren't on the Developer Preview programme.