how is it possible that can HR diff go down? thought it only went up
Because people are leaving the network, and when hashing power drops, less coins are found that hour, so the system changes difficulty to ensure x-coins are still found per day. EG, if we all left, the difficulty would drop fast. If we all came back, it would instantly rise as soon as we started generating massive amounts of coins.
https://blockchain.info/charts/hash-rate(That shrinking line to the left is us GPU miners... The rising line on the right, is the ASIC's being added to the network.)
BTW, that is a potential attack... If the difficulty drops real low, then everyone instantly comes back, everyone with these high powered machines will find coins faster than the system can calculate and adjust UP the sensitivity. Thus, making more coins available that day than should have been created. I would be willing to bet the ASIC guys will attempt this in the future.
Not to mention, when you have all that processing power in one location, and it goes down, that makes it a miners heaven for us little guys. Because of that drop in difficulty.
The ASIC's are touchy... look at the charts that show hashing power of the entire network, they keep crashing. The pools show similar results, as they crash, and the pie-slice for them drops down to a fraction of what it is, when they are alive and actually processing. I am not sure if it is hardware failures or network failures from DDOS attacks, but it is something real. (Hardware and network errors for ASICs have been reported. They occasionally have to "shut down" and reboot, to continue mining.)
