כִּ֛י אִם־מָאֵ֥ן אַתָּ֖ה לְשַׁלֵּ֑חַ וְעוֹדְךָ֖ מַחֲזִ֥יק בָּֽם׃
Denn wenn du es weigerst, sie ziehen zu lassen, und sie ferner zurückhältst,
Rashi on Exodus
מחזיק בם means WILT KEEP HOLD OF THEM, as in, (Deuteronomy 25:11) “and layeth hold of him etc. (...והחזיקה ב)”.
Haamek Davar on Exodus
And continue holding them. Although you have lightened the enslavement, nevertheless, you are still holding them as slaves and making them work.