These futures usually occur when ς is preceded by α or ε and these vowels are not preceded by a syllable long by nature or position. Here ς is dropped and -άω and -έω are contracted to -ῶ. When ι precedes ς, the ending is ι- (ς) έω which contracts to -ιῶ.
b. ἐλαύνω (ἐλα-) drive has Hom. ἐλάω, Attic ἐλῶ.—καθέζομαι (καθεδ-)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].