Twitter selective tweet blocking. What exactly does that mean?

There's a bit of a twitstorm brewing over an announcment made yesterday on Twitter's blog outlining a new capability the company has built into its platform. This capability will allow Twitter to selectively block content to particular countries in order to comply with local laws. Here's the bit that has some commenters worried:

Starting today, we give ourselves the ability to reactively withhold content from users in a specific country —   while keeping it available in the rest of the world. We have also built in a way to communicate transparently to users when content is withheld, and why.

Mark Gibbs has written this piece on forbes.com which suggests that Twitter will implement this blocking scheme through the use of realtime filtering, saying "given that over the course of 2011 the number of tweets per second (tps) ranged from a high of almost 9,000 tps down to just under 4,000 tps, any filtering has got to be computer-driven." However, if you read the full Twitter blog post you will see that this is not at all what the company has in mind. In particular:

We haven't yet used this ability, but if and when we are required to withhold a Tweet in a specific country, we will attempt to let the user know, and we will clearly mark when the content has been withheld.

One wonders whether Mr Gibbs bothered to read beyond the first three paragraphs, because if a system of filtering was being employed "starting today", then it would in fact be in constant use. What Twitter has done is to build some flexibility into their platform which would probably not have sparked such controversy had it simply been there from the very beginning. If it is sensible to do so, they may now selectively block tweets to certain countries on demand. Previously they would have had to block a tweet globally due to technical limitations, so if anything this is a positive step. Twitter might not ever use this new capability, but now they can and we will expect them to weigh the pros and cons and to exercise their best judgement. Some have suggested that the announcement of this new capability will place the company under greater pressure from governments to block tweets. I find that hard to believe. I have no doubt that Twitter is already under enormous pressure from some oppressive regimes. In the past they would have simply ignored the pressure at the risk of a nationwide block of the service. Whereas previously Twitter would have shrugged and cited technical limitations, now they can shrug and state moral objections. I think the company has made a minor communication error. They should have said "retroactively" instead of "reactively". The latter suggests that it is the platform which is reactive. There are however plenty of other clues in that blog post that point towards retroactive human intervention. That is, if you choose to read that far.

UPDATE: The Twitter blog post links to some other materials, including this very helpful (um) help page: https://support.twitter.com/articles/20169222. Here we clearly see that it is only upon receipt of a valid request for a takedown that any content will be withheld. As you were.