On May 31, 2013 at 7am, [greatfire.org] observed that searching for keywords that are normally blocked, for example, "六四事件" (June 4th incident), surprisingly returned some results and no censorship notice. This temporary lift of censorship ended at 9am but started and stopped a few more times into early afternoon, as if literally somebody was flipping a switch on and off at Sina headquarters.
To understand what is happening you need to be familiar with Sina’s various censorship methods. We observed and reported last year that Sina had implemented new tactics to censor particular keyword searches. Just days before the June 4th anniversary, Sina is again tweaking its censorship mechanisms.