The U.S. Sentencing Commission modifies a 2006 guideline, U.S.S.G. Section 1B1.13, Reduction in Term of Imprisonment as a Result of Motion by Director of Buereau of Prisons (Policy Statement) by providing examples of extraordinary and compelling reasons. http://www.ussc.gov/2007guid/may2007rf.pdf
The guideline implements Title 28 U.S.C. Section 994(t), a section of the law that allows the Sentencing Commission, under general policy, to provide a means to reduce a sentence of an inmate, upon motion of the Director of the Bureau of Prisons under Title 18 U.S.C. Section 3582(c)(1)(A). The court may reduce a term of imprionsment, if after considering the factors set forth in 18 U.S.C. Section 3553(a) the court determines that either 1) extraordinary and compelling reasons warrant the reduction, or 2) the inmate is at least 70 years old and has served at least 30 years in prison; AND the inmate is not a danger to the safety of any other person or the community and the reduction is consistent with the policy statement.
Keep posted for more particulars on what is compelling and extraordinary!