In this episode I asked Tim Schittekatte to explain me how the price cap on gas could be implemented.
Tim told me about a price cap could activated under the condition of flow interruption of russian gas (and not before this event), with an approach that avoid Russia claiming for a preventive aggression by EU.
The cap could be applied on the TTF platform where spot transactions are closed and because the TTF price is the common index for the long-term contracts.
Another option (see paper) is to put the cap only on the volumes that the gas TSO buys when the supplier in unbalanced between the entry and exit nominated volumes.
Supplier could then avoid to directly buy the gas needed on the TTF spot price, paying to the TSO the unbalance price (capped) for the gas that the TSO bought to cover the unbalance.

Here the link to the paper that Tim cites during the interview.
https://www.eprg.group.cam.ac.uk/wp-content/uploads/2022/06/combinepdf-10.pdf