You can print flexible using this upgrade with Smart Extruder +. Normally, flexible filaments wrap around the gear. This part closes that gap and pushes the flexible filament to where it needs to go. When I describe this part to someone, people ask me, "Does it affect the regular PLA prints?" No, it does not, depending on my 2 years of experience. The profile provided is mainly for NinjaFlex, but it worked for all flexible filaments I have tried.

USING AN OLDER SMART EXTRUDER?
The older version of the part for Smart Extruder.
https://www.thingiverse.com/thing:1224851

Important notes and warnings:

Opening inside your extruder will void its guarantee.

Sadly, MakerBot Print still doesn't support that much setting to be changed to print flexible. You can experiment with that. The only way to use it is to use MakerBot Desktop, which sadly doesn't support the newer printers. So, this is only for the 5th gen MakerBot Printers, for now (9th June 2017)

I had extruder issue which might be related to using this part, might have caused the gear to be worn out. Though, that problem only affected the filament jam detection, but not the printing. I have been using this part on that extruder for one year. I am not sure if the problem was caused because of using this part, but still, I am planning to design a new version which doesn't press on the gear much.

The photos are from my previous design for Smart Extruder. Now, I don't have any photo of this new version. I will be adding photos of this version shortly.

Known issues:
-It is suspected that because Tough and experimental extuders have a stiffer spring, some semi-soft materials like NinjaFlex Cheetah might fail to print by deflecting this part.

-If you don't use MakerBot True color PLA, it might create a part with slightly different dimensions, resulting with a part that is not working.