microSHIFT on front gears, how does it work?
Microshift is a brand. They are Taiwan base.

Most of the twist shifter operates in similar manner as you described. They are for trimming as Batman said. But in fact it is made from the same machine and parts to keep the cost down.

It is all about how to set up the front derailleur in your system, and find a spot for each appropriate gearing. i.e. avoid chain rubbing on the front derailleur

There is one thing that I like about twist shifter, is that when you shift up, you can go slightly higher than where it suppose to be on the front chainring. This helps your chain to shift up easier and faster.