I was reading a book on Japanese firearms in the feudal era. When the Portuguese brought firearms to Japan in 1543, the Japanese made various adaptions to them.
One such adaption listed was the addition of a helical main spring. What exactly is this device, and what advantage might it provide?
A "helical mainspring" is just a kind of coiled spring. The Japanese Tanegashima was a "snap matchlock" based on the Portuguese - snap matchlocks used a spring mechanism to swing the serpentine (a lever holding the match) to contact the flash pan (i.e. you loaded the matchlock, filled the flash pan with priming, lighted the match, aimed, pulled the trigger, the serpentine touched the match to the flash pan, the priming ignited and traveled through the touch hole, setting off the actual charge).
The use of the spring helped to automate the whole process, and meant that you could fire it more quickly and accurately (although loading a bullet and charged down a rifled barrel was no joke). Descriptions of the Portuguese models for Japanese matchlocks is included in The Defences of Macau 176.