I stumbled on this the other day, basically it is the fat stored in skeletal muscle fibres, AKA marbled meat.
It appears it is the primary fat source for extended exercise sessions, is found in endurance atheletes, particularly on high fat diets.
I expect this is one of those things grouped into the "Fat Adaptation" bundle, it is kind of like the Fat equivalent of Glycogen. Seems that the fatty acids from adipose tissue are primarily burned when resting or light exercise, it is the IMTG you rely on when you put the foot down.
So to get the best performance on the competition day you need to ensure your IMTG is at max levels as well as having the Glycogen stores full.

Some Links:
Intramuscular fat - Wikipedia, the free encyclopedia
High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise
Muscle Fuel Sources | Blog
Influence of prolonged endurance cycling and recovery diet on intramuscular triglyceride content in trained males
Bodybuilding.com - Intramuscular Triglycerides (IMTG) As An Ergogenic Aid.