edit:
There is NO 'mastering how' to use a Flexi.
The MECHANICS of a Flexi lead does NOT allow you to reel your dog back to you period. The string will not retract in when the dog is at the end of it pulling.
Your dog hits the end of that flexi lead at something the button does NOT reel him back in. You are left trying to wrap your fingers around a thin string trying to pull the dog back to you.
Have a family member call, tease, have food, whatever, while the dog is on the flexi at the end pulling toward the family member.
Now try and reel him back to you. You will quickly find how a flexi does NOT work. And IMHO better to find out with your family member than at a CG meeting up with another dog that is wanting to fight trying to kill your dog.