Bosu balls are extremely popular in core development classes. They provide instability during exercise, which may increase core muscle activation to stabilize the spine. A study from University College in Sogndal, Norway found that performing repetitions of sit-ups with the Bosu ball placed in the small of the back increased activation of the rectus abdominis by nearly 25 percent compared to standard sit-ups. Placing the Bosu ball under the feet during sit-ups did not increase abdominal muscle activation compared to normal sit-ups. Bosu balls placed under the low back can increase core muscle activation during high-rep sit-ups.
Source: Journal of Strength and Conditioning Research, 28: 3515-3522, 2014