“Second Hand Low; Third Hand High” is a general guideline, but there are exceptions.
When second hand has a SOLID SEQUENCE of three or more honors, play the TOP honor (the one you would have led in standard leads), thus Queen from QJ10(x) (x) and 10 from 1098(x), etc. This is true even when you are playing upside down signals. Honor sequences trump upside-down count. Partner needs to know when you have those sequences.
Partner is expected to understand from the bidding (and play of the hand to that point) that your honor is NOT singleton. (Obviously if you have a doubleton honor, you play top again for same reasons—Queen from QJ even if playing upside down.)
What about when you have two honors, e.g., KQx or QJx(x) in a suit? Unfortunately, it is complicated:
1) In THEIR trump suit, it is almost never correct to "split" your honors (e.g., King from KQx in second seat). The opponents do not always take the safety plays that they should. When you have QJxx in their trump suit and you play the Queen or Jack to "force" a higher honor, Declarer will know what to do. If you play LOW smoothly, Declarer may fail to take a safety play and you will get TWO trump tricks. Splitting your honors means you'll never get more than one. (And think how awful you'd feel if you split from QJx, Declarer plays Ace and partner's singleton King falls.)
There are exceptions to almost everything. For example, if Declarer has a singleton trump in dummy and has shown a six card or longer suit in hand from the bidding, it is OK to split your Queen from QJx rather than risking that opponent might finesse to 10 on first round of suit.
THINKING about the whole hand is inevitably superior to following guidelines blindly.
2) Most of the time, you do NOT want to split from QJx(x) as it will help Declarer more than partner. However, if you KNOW that partner has to have an honor in the suit from the bidding, it may be helpful for you to split to finesse whatever holding Declarer has. Late in the hand, when Declarer has no more entries to dummy, it is almost always right to split your QJxx to prevent Declarer from taking a really deep finesse. Declarer cannot get back to dummy even if s/he happened to start out with AK10 in the suit.
3) In a suit contract, it may be appropriate to split from KQx because you don't want to give away an extra overtrick. In no trump, it is often right to duck and await developments, but use your judgment based on what you see in dummy and know from the bidding.
4) When you have only ONE honor, e.g., Jxx or Qxx, it is almost always right to play second hand low. HOWEVER, if you can see that Declarer is eliminating suits and setting up for an end play on your partner, you MUST play second high to rescue partner as in the following hand.
Dummy: ♠Q97 ♥6542 ♦KQ3 ♣K3
You: ♠32 ♥J73 ♦9874 ♣J1064
Declarer opens 1♠. Dummy bids 2NT (Jacoby) and Declarer goes slamming. They end up in 6♠.
Partner leads the ♣Q. Declarer takes the ♣A in hand as you signal with ♣J (promising the ♣10) in case you have to take over guarding clubs later for partner. Declarer pulls trumps in two rounds, your partner playing the ♠8 then the ♠4. (You play trump suit preference with this partner.) Declarer then plays three rounds of diamonds, with everyone following. Declarer cashes the ♣K and your partner plays the ♣8—standard signals, showing an EVEN number of clubs remaining in her hand. Therefore, it appears that partner started with five clubs originally, three diamonds, two spades and three hearts. Declarer presumably started with five spades, three diamonds, two clubs and therefore, three hearts. Declarer now plays a low heart from dummy. You MUST PLAY THE ♥J.
If you play a low heart, Declarer will insert the ♥8 and your partner (who holds ♥K109 of hearts will be end played and have to give a "free finesse" or ruff-sluff to Declarer for the slam-going trick. By playing the ♥J, you insure that you side gets two heart tricks (whenever two tricks are possible).
When declarer has a nine cards or better fit, especially 5-4 or better but even 6-3 as in this example, the defense should always be on the lookout for the ruff-and-sluff or lead from a tenace endplay.