I think I can answer my own question now, I've found the necessary documents through my broker's website.
An index is tracked by a replication strategy. There are grosso modo 3 different replication strategies:
- Full replication strategy: The fund owns all the shares and bonds that constitute the index in its portfolio.
- Representative sampling strategy: As the name implies, only a representative sample of the index is effectively owned by the fund, only 80 to 90% of the most liquid assets are effectively owned in the portfolio.
- Synthetic replication: The fund contracts a SWAP that delivers 100% of the performance of the underlying index.
The method used by Lyxor ETF's is the last one. So they replicate the index with a certain basket of assets and handle all the differences between the replicated basket and the replica through the SWAP contract.
I couldn't find how the conversion factor between index value and tracker price is decided though.