Loop a resistance band over a pull-up bar and place your knee (or foot) into the band loop for assistance. Grip the bar with an underhand (supinated) grip at about shoulder width, engage your core, and pull yourself upward until your chin clears the bar. Pause briefly at the top, then lower yourself back down in a controlled manner. Repeat for reps.
Log your band assisted chin up (from knee) sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.