This is LNHANCE, a soft fork proposal from Brandon Black that essentially does CTV and LN-symmetry. Check out this PR https://github.com/bitcoin/bitcoin/pull/29198
InternalKey and CSFS are two little opcodes to work with signatures on the script stack. One puts the taproot internal key on the stack and the other lets you check a signature against that key. Together with CTV they let you build LN-symmetry in a different way than SIGHASH_APO. You get CTV covenants and massively upgrade lightning channels with the safest and most minimal consensus change.
APO always seemed a little sloppy to me. It introduces a footgun similar to SIGHASH_NONE. You see the guy that lost 0.8 btc from using SIGHASH_NONE last week?
Need to learn more about TR Internal Key. CheckSigFromStack make sense.
The rest I mostly grok. I hope to learn a lot about TR at TAB Conf.