This exercise targets the rectus abdominis perfectly. Here's how to do it:
Position the small of your back over the center of the BOSU ball and "wrap" your spine around it.
Grab the bar with your arms perpendicular to the ground. Keep your elbows locked.
Start the movement by "curling the spine" upwards as far as your range of motion will allow.
Make sure not to protract your shoulders as you curl up in an effort to raise the bar higher (remember, it's an ab movement, not a chest or shoulder movement).
Reverse the movement until you're flat on the BOSU. Pause a second before starting the next rep.