POMO is an abbreviation for "Permanent Open Market Operations" which is the scheduled buying or selling of Treasury and/or MBS securities by the Federal Reserve Bank of New York as mandated in policy decisions by the FOMC.
In short, the abbreviation POMO is simply a convenient way to refer to the almost daily dose of scheduled Fed buying in Treasuries. MBS market operations do not adhere to the same sort of schedule as do the Treasury operations. Whereas the Fed may be buying MBS any time of day, the Treasury POMO's are either scheduled from 10:15-11:00am or in rarer cases from 1:15-2:00pm.
While POMO technically refers to all of the Fed's permanent open-market operations, in MBS Live parlance, it will almost always connote the 10:15-11:00am blocks of buying as specified here:
More on POMO's: http://www.newyorkfed.org/markets/pomo/display/index.cfm
On a qualitative note:
We've observed on numerous occasions since the inception of Twist, the scheduled buying that typically concludes at 11am often times coincides with the best levels of the day for bond markets, all other things being equal. To oversimplify the dynamic: if the Fed's coming in as a guaranteed buyer of a set amount of dealer's inventories, why not push prices higher ahead of the takedown?
This certainly isn't a hard and fast rule, but more often than not, there's at least a bit of volatility at 11:02am on POMO days, usually in a negative direction for bond prices.