There have been some good responses to your query. However, theres
been rather a lot of fancy words bandied about. forget these, What
is in fact hapening is your stretching metal, using a hammer and
supportive block of preferably hardened steel.
to make the fastest progress, just have a go on some 1mm thick
aluminium or copper.
I dont do that kind of work but yesterday, i thought Id have a go.
took an ali disk 2in dia, marked it with a pen into 4 quadrants, took
an ordinary carpenters cross peen hammer, and had a go. 20 mins later
I had your saddle shape. you need to do the same.
Experiment on metal that costs you virtually nothing, only your
time. And its a learning opportunity for you. Dont be afraid to hit