What are Forward Blocks? Mark Friedenbach says "Bitcoin’s Block Size Can Be Increased Without Hard Fork".

Can someone ELI5 what are Forward Blocks?
From this week's Bitcoin Optech


  One well-known method for soft forking an increase in the Bitcoin
  block size is extension blocks—a data structure that’s invisible to
  nodes that haven’t upgraded to the soft fork and so is not subject to
  their historic limits on block size. By itself, this is an undesirable
  method for increasing block size because preventing old nodes from
  seeing the transactions in the extension block also prevents them from
  being able to enforce any other consensus rules on those
  transactions—such as rules that prevent a malicious user from spending
  other users’ bitcoins or from creating more bitcoins than allowed by
  the 21 million bitcoin subsidy schedule.
  
  However, one doesn’t need to increase block size to increase the
  amount of data that can be added to the block chain per minute—it’s
  also possible to increase capacity by increasing the frequency of
  blocks (reducing the average time between blocks). A method for gaming
  Bitcoin’s difficulty adjustment algorithm—called a time-warp attack—is
  well-known among experts and has been used successfully in
  demonstration attacks against Bitcoin’s testnet and real attacks
  against altcoins. (Note: although Bitcoin is technically vulnerable to
  this attack, it’d be a slow attack that would give the userbase a
  significant amount of time to respond.) By itself, increasing block
  frequency is also an undesirable method for increasing capacity
  because shorter block intervals increase the effectiveness of miners
  with large amounts of hashrate and so is likely to increase mining
  centralization.1
  
  Perhaps disproving the saying that “two wrongs don’t make a right,”
  this talk describes a novel way of combining extension blocks and the
  time-warp attack to allow both upgraded nodes and old nodes to gain
  the same capacity increase and see all the same transactions for
  validation while simultaneously slightly reducing mining
  centralization risk. Upgraded nodes would validate one or more
  extension blocks (called “forward blocks”) that provide additional
  block space with a centralization-reducing 15 minute average interval,
  but the upgraded nodes would also restrict the time stamps in legacy
  blocks to ensure a permanent (but limited) time warp attack increased
  the frequency of legacy blocks enough to allow them to include the
  same transactions that previously appeared in the forward blocks.


Video / Transcript / Paper