How to avoid the runaway bur?

instead, he had a handle similar to a graver handle in which he
could chuck a standard burr. No "traveling 'round the prong" there!
He used this and a graver to cut all his seats and notch prongs.
Alas, the first store let him go because he worked too slowly. 

Using flexshaft is commercial adaptation of setting technique.
Classical approach is to use flat graver.

Prong can also be undercut using jeweler saw, or small file. Bur held
in a pin wise is also very good technique, especially for cleaning
the cut.

I never use commercial settings. I like to make a collet, and cut out
the prongs with jeweler’s saw. Before I cut the prongs, I cut a
bearing seat for the stone, with a bur. That is another way to avoid
runaways.

Leonid Surpin

I thought I would add something from my perspective as a dentist and
a Dental educator:

Eye hand coordination is a skill that most people can learn. It
involves Hours of practice: good lighting, magnification, comfortable
posture, and For some no drugs like coffee!

Using a 400,000 RPM handpiece requires focus and a light touch. Even
so, Using a carbide bur on metal such as an old silver-mercury
filling or gold Crown, I sometimes get some kickback: but I am
prepared for this by my bracing on a purchase point. When using my
non-dominant hand to control the Handpiece, I am even more careful.

Just as an aside when I cut off a porcelain crown or a base metal
crown, I will use a diamond bur, it is slower, but almost never kicks
back.

Charles Friedman DDS
Ventura by the Sea

Just as an aside when I cut off a porcelain crown or a base metal
crown, I will use a diamond bur, it is slower, but almost never
kicks back. 

Hence my preference for a separating disk to cut prongs rather than
a bur. YMMV