I was last night years old when I learned about signing Bitcoin transactions using MPC keys

Like Multisig, but a single sharded key that never comes together reducing the attack vector on signed transactions.

The best way to describe it is, imagine I wish to perform a multiply operation, for example 132 x 72.

Now imagine 72 is the UTXO (public) transaction.

And 132 is the secret key.

Not only do I not want any signer to know this number (132), I don't want it to ever been constructed, so I get three people to do 3 sums:

1st person: 100 x 72 = 7,200

2nd person 30 x 72 = 2,160

3rd person 2 x 72 = 144

I can then add the three outputs together = 9,504

I have signed (added together for this example) the message, without ever revealing or even constructing my key (132).

This is an analogy, key sharding and signing are more involved, but this gives a general concept.