The ONLY time I use unstable surface training.
Generally, I loathe training on unstable surfaces. Squatting on a BOSU or dumbbell bench press with your back on a stability ball is not only a complete waste of time but also potentially very dangerous.
However, the one exception I make for unstable surfaces is with push-ups. Specifically, push-ups on an upside down BOSU ball or gymnastic rings.
Both of these options can significantly increase the stabilization demand of the shoulder stabilizers and trunk musculature without being so unstable that they drastically reducing force production or make the exercise unsafe. This create a unique stability focused challenge for the client without turning the gym into a circus act.
I opt for these variations as advanced progressions in this order:
Incline or Band Assisted Push-Up
⬇
Push-Up
⬇
Decline Push-Up (6-18 inch elevation)
⬇
➡Option 1 for Load Focused Client: Loaded Decline Push-Up
Or
➡Option 2 for Stability Focused Client: Bosu Push-Up ➡ Ring Push-Up
