The mechanism of a glycosynthase is similar to the hydrolysis reaction of retaining glycosidases except no covalent-enzyme intermediate is formed.   Mutation of the active site nucleophile to a non-nucleophilic amino acid prevents the formation of a covalent intermediate.  An activated glycosyl donor with a good anomeric-leaving group (often a fluorine) is required.  The leaving group is displaced by an alcohol of the acceptor sugar aided by the active site general base amino acid of the enzyme.