Two discoveries led to the development of glycosynthase enzymes.  The first was that a change of the active site nucleophile of a glycosidase from a carboxylate to another amino acid resulted in a properly folded protein that had no hydrolase activity. 
The second discovery was that some glycosidase enzymes were able to catalyze the hydrolysis of glycosyl fluorides that had the incorrect anomeric configuration.  The enzymes underwent a transglycosidation reaction to form a disaccharide, which was then a substrate for hydrolase activity.
The first reported glycosynthase was a mutant of the  sp. β-glucosidase / galactosidase in which the nucleophile glutamate 358 was mutated to an alanine by site directed mutagenesis.  When incubated with α-glycosyl fluorides and an acceptor sugar it was found to catalyze the transglycosidation reaction without any hydrolysis.  This glycosynthase was used to synthesize a series of di- and trisaccharide products with yields between 64% and 92%.