If they managed to Side Mount you, they will most likely want to get on top of you in the Mounted position. One way they do that is by staying low to hold you down, and slide their shin/knee across your abdomen, and then plant their knee down.
In that moment that their knee touches the ground, that's when you take your elbow and push their other leg up and slide your leg out to wrap over theirs. From there, it all the steps for the Elbow Escape.
It is that moment when their knee touches the ground that their other leg for a split second becomes light. When it is light, that's when you push it down and slide your leg out. If you wait too long, you missed the opportunity.
So timing is the important piece, here.
Side note - so let's say you missed the timing, and they have Mounted on you. Do you remember what to do? Try to Trap & Roll, and if that doesn't work, use one of the Elbow Escape techniques from the Mount. So, there's always something you can do.
Follow me for more Martial Arts Instruction on social media:
Twitter - @AustinKungFu
Instagram - @AustinKungFuAcademy
Facebook - Facebook.com/AustinKungFuAcademy