Just like the term 'end play', second hand low is borrowed from the world of bridge. Many euchre players tend to think that second hand low only refers to cards played in the trump suit. In reality, it refers to any play where your right hand opponent leads a card. You then play a low card that will not take the trick. The basic premise is that you're saving your higher card for use latter in the hand. There are many different ways this strategy can be used to your benefit. It can be used any hand where either when you or one of the opponents named trump. In either case, one of the main objectives is to give your partner a chance to take the trick. Another reason would be that your right-handed opponent is hoping to promote another card that they hold. Playing low would stop this from happening.
In this example, the dealer sits in East position and turns-up the ace of diamonds. No one orders and naming trump is passed back to the dealer. As he already holds the left and the king, picking the ace will give him three trump. This hand is good for two sure tricks. He just needs a little help from his partner and should have no problem making a point. He discards the 10 of clubs.
South starts the hand and leads the ace of clubs. West follows suit with the 9, North throws off the king of hearts. The maker(E) takes the trick with his king. They have their first trick.
Now the maker(E) leads the left. He is trying to pull out the right and promote his ace. This lead also tells his partner he doesn't hold the right. South, planning ahead, plays second low by throwing the nine of trump. West shows void in trump and North plays the queen. Now they have two tricks.
Seeing that his partner(W) doesn't have any trump but both of the opponents are still following, maker(E) has to find a way to let his partner get a trick. Leading a heart is asking for trouble due to the fact that next is always short one card. The jack of spades seems to be his only hope. South, in sticking to his plan, throws off the queen of clubs. West plays the king and North takes the trick with the ace of spades. The opponents have one trick.
Now North leads back the 10 of spades. The maker(E) knows his partner has no trump and likely doesn't hold any spades because he played the king on the last lead. He has to trump in. Now he's down to one last hope, trump the spade and maybe his queen of hearts is good.
First seat has a different plan. He had set this up early in the hand by playing second low. He over-trumps the ace and his king of clubs completes the euchre.
In this particular hand, the maker was likely to be euchred no matter what; still it shows how second hand low works and the thought process behind it.