Sitting and cutting circles/arcs can cause the elbows & knees instead of the smooth continuous cuts you want because you are stopping and re-starting the cut several times.
First is to draw out the scroll or arc on the metal exactly the way you want it.
Then follow that line exactly.
Check your drawn pattern line from a couple different angles to make sure it looks right. Sometimes it can look great from one way. Then turn the work and ,,not so great. Re-draw it.
When cutting, don't try and cut to the absolute limit of what your arm and wrists will tollerate. Just cut the arc smoothly for a distance and then stop.
Turn the work.
Now start again,,BUT,,back up with the chisel on the already cut line and start again.
DON'T stasrt cuttin right at the point where you ended.
You won't be able to smoothly and cleanly extend the arc that way as a rule. You will leave a kink or elbow in the line when proceding. Backing up and retracing your already cut line gets you started on the orig arc again and then you cut right into the same arc in the new cut.
After awhile the process goes pretty fast, but don't try for speed with anything when leaning engraving.
It'll just leave you with poor quality work, frustration and wanting for some sort of shortcut to sucess.
There isn't any.
Just practice.
You'll get there.