No you are wrong ;)
"Helpers line" can be use before the first click of a function, after the first click of the function and before a last click of a function!

So you can use it for place the center of the arc, and after for place the first point and also for the second point ;)

Just click and move without release then release
When you want draw your point just click and release ;)
Just make some trainning! It's the more powerful tools of Moi for put with infinite precision something!

Say you must have a center arc at the middle of middle of the 2 lines top from the 2 volumes ;)
Helps line can help you ;)