Remember to ALWAYS use a harness if you have your dog on a flexi lead, never a collar.
Use a harness that wont pull on the neck, if you use Flexi lead with a collar and the dog runs 5-8 meters then pulls on the leash hard, it can damage the neck, but if you have a good harness that doesn't pull/let weight on the neck then it is much much safer and better, he wont hurt then or damage his neck since yorkies have so small thin necks.
Do not always use a flexi, only use it when you are letting the dog run and you don't wanna have him 100% free, do not use flexi to tie the dog, and never let him chew the lead cause then it breaks easily
I use flexi leads much when I'm letting Fido run or when we play fetch, he loves it, but for example when we walk where there are many people I use 1-2 meter nylon or leather leash and let him heel.
I also use the Flexi lead much when I'm out jogging