The Horner-Wadsworth-Emmons reaction is a widely used olefination reaction in which a phosphonate-stabilized carbanion reacts with an aldehyde or ketone to form an alkene.  In the standard HWE reaction, the phosphonate ester contains alkoxy substituents (typically methoxy or ethoxy), producing an E-alkene as the major product.  In 1983, Still and Gennari reported that simply substituting the more electron-withdrawing 2,2,2-trifluoroethoxy groups on the phosphonate component reversed the stereoselectivity to produce predominantly Z-alkenes.  The use of a non-coordinating counterion was also found to be critical for high levels of Z-selectivity; this is typically accomplished by using a base with a potassium counterion in the presence of 18-crown-6.  Taken together, this protocol is known as the Still-Gennari modification of the Horner-Wadsworth-Emmons reaction or, alternatively, as the Still-Gennari olefination.  
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Although the mechanism of this reaction has not been fully elucidated, it is speculated that the use of an electron-deficient phosphonate accelerates elimination of the oxaphosphatane intermediate.  This renders the initial addition of the phosphonate-stabilized carbanion to the aldehyde effectively irreversible, leading to Z-selectivity in analogy to the standard Wittig reaction.