A pretty simple concept to understand, but also a simple concept that I think a lot of people do not use. Before beginning any program or selecting any exercise, the pluses and minuses should be weighed out. A particular exercise may look good on paper, but does it put you at risk for further injury if you’re already injured? Yeah, crunches or sit ups may may help develop a six pack ( you will not hear that from me) , but also at the expense of ruining your spine. I watch people do certain things in the gym and wonder how they do not crawl out the door on their hands and knees. And then I watch people get so little out of their exercise. Standing on one foot on a Bosu Ball curling 10lb dumbbells for example. You have a greater chance of rolling your ankle or falling off that stupid thing than you do of getting any real training effect. If you want to work on balance or stability, get on some flat ground and do some single leg work. Just because something looks “cool” or hard to do, does not mean that it will serve any real benefit. When in doubt, get back to the basics.