The conception: When partner leads a suit against a suit contract, you always play high-low from a doubleton.
The truth: If partner leads the A of a suit, then you signal high-low (unless you hold Qx*), but your signal is NOT a count signal. The reason you play high-low is to show attitude. With a doubleton, you want partner to continue that suit. You would also want partner to continue if you hold the Q of the suit so from Q82 you would play the 8 and then the 2 playing standard carding.
If partner leads the K against a suit contract, and you hold 82, you don't like the suit. Play low. If you hold the J (say, J82) then you can encourage partner with the 8 since partner will also hold the Q.
What about getting a ruff? Picture this holding.
|CONTRACT: 4|| 9763
Partner leads the K against 4. If you play the 8 and declarer ducks (which he should), partner will think you have either the J or A and continue the suit. This will cost your side its second spade trick. Declarer can win the jack and draw trump. This is an "attitude" situation and you don't like spades, so play low. Partner can switch and hope to get his second spade trick later.
*With Qx you should play low when partner leads the ace as the Q promises the J.
You can watch Larry talk about Signaling in this webinar: https://www.larryco.com/bridge-store/detail/larry-teaches-defense-receiving-signals-webinar-recording-aired-62520