Extracts:

“For every new spending condition a developer might want to create, a lot of time has to be spent making sure it is correct and sound under all circumstances…”

“Bitcoins are generally locked by scripts which encode what kind of conditions need to be met in order to spend the coins. …the witness script encodes the conditions that need to be met in order to spend a bitcoin. They commonly include public keys. The witness is the data required to satisfy the spending conditions. Witnesses commonly include signatures corresponding to the public keys.

To spend a coin locked with a witness script, the transaction spending it must include a valid witness. The witness and the witness script are evaluated according to the Bitcoin Script rules.”