Why?

To provide the option of creating a superstar

What does he do?

He is attached to one player at the start of a season. At the end of that season, that player gains one extra SL.  

So, a SB who has reached level 5 becomes level 6, an Age II player who has reached level 16 becomes level 17 (but then ages in the normal way down to level 14)

Issues to consider

This coach cannot work with GKs or SWs.

Advantages/Disadvantages 

If you attached this coach to the same player for his entire career, his skill levels could look something like this at the end of each session after he gets his bonus:

SB - 6

App - 12

Age I - 18

Age II - 20

Age III - 21

Age IV - 21

Age V - 20

Age VI - 18

Age VII - 15

Age VIII - 11

Age IX - 6

Would we want a superstar to be ineligible to be a captain? An Age V level 19 captain is a daunting prospect but you'd have had to make a huge investment over a number of years to get a player to that level