Speed/Turbo casting has been in UO since the beginning, and technically you can cast one spell immediately after another even if you have 0 FCR.
The way casting recovery in UO works, is that it is the time from when you begin casting a spell, until it is ready to be released.
This is why if you watch old school mages fight, many times you will see them cast a spell, and wait a second or two to release it. The recovery time takes place after casting, not after release.
If you cast spells immediately and back to back this is the sequence
Cast->Release->Recover->Cast->Release
To Speed Cast
Cast->Recover->Release->Cast->Release
No Delay between release of the spell and casting of the second spell, making
interruption much more difficult
This doesnt work with the new targeting system in the EC since it releases spells immediately once they are finished casting.
This also doesnt work if you use last target queueing in UO assist, unless you wait to target the first spell.