Ashtama Shani represents two and half years time when Shani transits through the Eighth Rasi as counted from Janma Rasi (where Chandra is situated in horoscope).
Presently in the transit of September 2009 to November 2011, Shani is transiting through Kanya Rasi and those who are born with Chandra/Moon in Kumbha Rasi will be going through Ashtama Shani.
Ashtama Shani generally gives sorrow due to children, loss of wealth, treachery from friends/near-dear ones, new diseases or resurfacing of old/chronic diseases, unexpected quarrels.
During this period, the concerned Rasi people may face unwanted expenditures, legal wrangles, involvement in disputes/court cases, tiresome travels, unexpected transfers(for employed people), accidents and in some cases surgery may also be involved. In some cases this period may see death in the family also.
The above effects are general and may not apply to all. The effects of Ashtama Shani may be of greater or lesser magnitude depending on the position of various planets in the horoscope. Generally for persons born with Chandra in Makara and Kumbha Rasi may not feel the intensity of Ashtama Shani to that extent because Shani is owner of these Rasis.
Read more on the transit of Saneeswaran on 14th Nov 2011 into Thula Rasi starting the 3rd phase of Sade Sati for Kanya(Libra),2nd Phase of Sade sati for Thula(Libra) , the 1st phase of Sade sati for Vrischika Rasi (Scorpio), Ashtama Sani for Meena(Pisces) and Ardha Ashtama for Karka(Cancer).
Further, if a person has Shani in a very good position in the horoscope with all friendly features etc., the period governed by Ashtama Shani may pass off somewhat smoothly.