Structures around the ankle deal with a lot of stuff just on daily basis. Throw a little sport into the mix and there’s some serious physical requirements going on down there. For example the forces exerted on the Achilles tendon during running have been predicted to be up to 10x body weight (https://www.ncbi.nlm.nih.gov/m/pubmed/7132650/) (and some studies have suggested even higher).
Now imagine that you’re running and getting a good mid-foot/toe strike with the landing force being absorbed by the Achilles tendon (among other things), going through mid-stance, and then the force ramping up again as you drive off… but then instead of landing on your toes your foot hits a divot.
There’s no support on the outside of your foot and it starts to roll, instead of that force being absorbed by your Achilles tendon it’s now all going through the structures on the outside of your ankle.
The peroneal muscles try to take that force and pull your foot back out, but they’re just not strong enough and your first line of defence fails. It’s then left down to the ligaments on the outside of your ankle to try and absorb nearly 10 times your body weight. And unfortunately a lot of the time that’s simply too much to deal with.
The end result is a an ankle sprain. There’ll be many times when you can pretty much shrug it off – it’ll feel a little sore for a few minutes, but after a while you forget it even happened. Other times you’ll know that’s it and that training session or match is over for you.
The biggest problem comes after the injury though, as in most minor instances of sprains you’ll be generally pain free within a matter of days after the injury. Unfortunately that doesn’t necessarily mean that it’s healed. It takes around 6 weeks for the body to even complete the process of repair with scar tissue (collagen fibres). Following that it can take several months to restore the full strength of the ligament appropriate for your sport.
In the meantime, because they’re pain free many people will return to sport. It’s perfectly understandable and I’m no exception, if everything feels ok then why not play?
The reason is that the ligaments are still healing – this is putting them at a higher risk of further injury. Plus when an ankle is sprained it’s not only the ligaments that are damaged, other structures including nerves usually suffer damage as well. You’re proprioception (your body’s sense of where it is) is also affected, which means you’re less aware of when your ankle is going into ranges of motion that might cause an injury.
In summary not only are you running around on something that’s still damaged, it’s also more unstable and more likely to go beyond it’s safe range. The end result being further, more severe damage, to the ankle.