My understanding... is that Foundry in this case (or any miner) would need to effectively mine 7 blocks consecutively (before any group of miners could collectively) without broadcasting any of them.

Then after the 6th block in the 'honest' chain is broadcast (6th block or 'confirmation' after a transaction occurs) Foundry would have to suddenly broadcast all 7 blocks which would not include some or many of the transactions that just took place 6 blocks before.

This would basically be... a 're-org' after 6 blocks (how long it takes for full confirmation.)

Basically, just mine 7 consecutive blocks in sequence, without announcing any of them, before 7 blocks can be added to the chain consecutively (by any number of miners). Then 'suddenly' announce the 7 blocks, potentially not including transactions that people had waited 6 confirmations for.

In order to 'double-spend' or 're-spend' UTXOs that are now unspent.

I believe that's my understanding.