UDCA (upside down count and attitude) has some advantages over other methods (standard, odd-even, Lavinthal, etc). Principally, it avoids wasting a high card to signal positive attitude in a suit and it reduces the possibility of false carding by declarer.
Here’s an example from bridgebum:
A defender might like to show count or attitude in a suit like KT82, but a standard count or attitude signal here would require the 8. This could be a valuable card. Theoretically, it’s more practical to discard a high spot card from a lousy holding like 872.
And another one from Marilyn Hemenway:
You hold: !sxxx !h109x !dAQx !cxxx. Dummy is !sKQ9x !hxxx !dKxx !cAxx. The contract is 4!s. Partner leads the !h King (which in your methods asks for upside down count), you play the !h 10[showing odd (3 hearts). Partner continues with the !h Ace on which you play the !h 9 [has to be your highest remaining card] which is suit preference suggesting a switch to the higher ranking suit….in this case diamonds. Partner dutifully switches to a small diamond and you get to cash two diamond tricks to defeat the contract one trick.
When not to signal?
The general principle is to not signal if doing so will help declarer more than the defenders.
Case 1: You judge that you hold virtually all of the defensive cards, that partner will very probably never gain the lead. Do not signal. Partner, on the other hand, knowing the situation, should consider signalling honestly.
Case 2: Signalling against a slam is very dangerous. Make declarer work as hard as possible. Don’t give him any indications.
So is UDCA worth learning?
Many pairs do fine with standard signals. UDCA gives a slight edge (but not if partners forget they are doing it). I also think learning new and useful conventions helps sharpen the mind, so enough reasons for me to learn it. I just need to convince my partners to try it!