The first few times you try this method you don't actually go anywhere.
Just back and forth over a few hundred metres until they start to get the idea.
As they get the idea, they walk further keeping their attention on you and so you get more distance.
As for reacting to cats / dogs / squirrels / blowing crisp packets etc, that's a whole different set of training altogether.
The WYDWL harness and other No-pulling tools such as the Halti and Canny collars are training tools.
Use them to stop the initial arm dislocating pull, then use loose leash training as well, until you can transfer them onto a normal collar or harness.
These tools are not designed to be used as a permanent solution to stop the pulling.
They are great though to give you some measure of control, while you work on further training.
Keep up the good work.