שמות ט:ב פירוש: רש״י והעמק דבר

מחזיק בם means WILT KEEP HOLD OF THEM, as in, (Deuteronomy 25:11) “and layeth hold of him etc. (...והחזיקה ב)”.

And continue holding them. Although you have lightened the enslavement, nevertheless, you are still holding them as slaves and making them work.