Core stiffness sounds negative but is exactly the opposite. Core stiffness is the the stability and strength in your trunk /core area.  You want this core stability because a strong stable core allows you  perform better overall in all of your daily activities, including exercise. Here are the moves: Start with one set and progress to two sets – 2  – 3 time per week on non-consecutive days.
Standing Band Presses:  10 – 15 slow reps on each side
Standing Band Torso Rotations 10 – 20 reps on each side
High Plank Walk Outs: 10 – 15 reps