Drift City is a bad game!
What does a Mittron Enhancer do?
A Mittron Enhancer can be used to modify an existing part into a more powerful version of itself. When you have obtained your Mittron Enhancer, go to the Parts Shop in the Driver Dome. Click on the tab that says "Modify." Select the part you would like to enhance. Put the part you would like to enhance on the left square, and the Mittron Enhancer on the right square. Click the Modify button. This will automatically deduct the modification fee shown at the top. Assuming you succeed, your part will be upgraded by 1 to 3 points and the name will change from X to X+y, X being the name of the part and y being the number of times it was enhanced (e.g. Shinoya Beta Booster+12). Although there is no hard-coded limit of the number of times this can be done to a part, the success rate is reduced after each prior successful modification.
Modification can fail however. The failure rate increases as a part is successfully modified, so after about 4 successful modifications, you will fail more often than succeed. That doesn't mean the item will be destroyed and unequippable, it just means that the process did not work and the item did not undergo any changes.
As of the Stage 2 update, parts can sometimes break if an enhancement fails. If a part breaks, it becomes unusable, but it can be sold on the Auction House or to the Parts Shop. Broken parts are uncommon below 7 enhances and become very common after 10 successful enhances. Broken parts can be fixed by using the appropriate level Repair Kit from the Parts Shop. Using a Repair Kit will remove all enhancements from the part however. Restoration Kits are available through the Drift Shop that can be used to repair parts and keep all previous enhancements up to +12.
You can also use a Mittron Enhancer to make Hyper Parts. When you "modify" a Crude Part, it becomes a hyper part. Hyper parts range from level 60 to level 80 and have special characteristics that differentiate them from other parts.