You've probably seen these in gyms. According to their site:
BOSU™ is an acronym for "both sides up". The BOSU® Balance Trainer can be used with the platform side either up or down for different types of balance challenge. The solid platform is 25 inches in diameter and the dome should be inflated until it is firm. Two recessed handles on the bottom of and towards the sides on the platform make it easy to turn over or carry. This hybrid fitness product has its genesis in the field of medicine, as well as balance, functional (oh no! you know we have been discussing functional training!) and sports specific training. Neuromuscular physiology, which helps to define human movement, provides the science that backs this remarkably complete approach to training. The BOSU® Balance Trainer offers a different means to make exercise more appealing and effective for average people, fitness enthusiasts and highly trained athletes.
Here is a study I have just seen that questions the use of the BOSU:
Effect of surface stability on core muscle activity for dynamic resistance exercises.
PURPOSE: To compare core muscle activity during resistance exercises performed on stable ground vs. the BOSU Balance Trainer. METHODS: Twelve trained men performed the back squat, dead lift, overhead press, and curl lifts. The activity of the rectus abdominis, external oblique abdominis, transversus abdominis/internal oblique abdominis, and erector spinae muscles was assessed. Subjects performed each lift under three separate conditions including standing on stable ground with 50% of a 1-RM, standing on a BOSU Balance Trainer with 50% of a 1-RM, and standing on stable ground with 75% of a 1-RM. RESULTS: Significant differences were noted between the stable 75% of 1-RM and BOSU 50% of 1-RM conditions for the rectus abdominis during the overhead press and transversus abdominis/internal oblique abdominis during the overhead press and curl (P < .05). Conversely, there were no significant differences between the stable 75% of 1-RM and BOSU 50% of 1-RM conditions for the external obliques and erector spinae across all lifts examined. Furthermore, there were no significant differences between the BOSU 50% of 1-RM and stable 50% of 1-RM conditions across all muscles and lifts examined. CONCLUSIONS: The current study did not demonstrate any advantage in utilizing the BOSU Balance Trainer. Therefore, fitness trainers should be advised that each of the aforementioned lifts can be performed while standing on stable ground without losing the potential core muscle training benefits.
Being on the BOSU or off it doesn't make a difference!