Electronic Fuel Injection Enhancer
The EFIE, which stands for Electronic Fuel Injection Enhancer, is used to modify the signal coming from the O2 sensor to the computer. This device has the same goal as the map sensor enhancer but acheives it in a different way. Some people use both of these at the same time in their system as well.
This device is a little harder to come buy then a map sensor enhancer and they are much harder to make so they are more expensive. They also seem to do a better job though.
I have seen a lot of people that will have EFIE in the listing title when they are actually selling map sensor enhancers so make sure you read the description before you buy one. In order for it to be an EFIE then it will be adjusting the signal from the O2 sensor, not from the Map sensor.