Legal Communication in euchre, page 6
Don't help your opponents, example 2
Almost anytime trump is named the maker wonders not only what cards may be out against him but who has them. If they're in his partner's hand, all is well. If the opponents hold them, having some idea of where they are could be helpful. As an opponent, it's your job to try and hide what you hold from the maker. This information often is the difference between a euchre and a point.
Look at the following example:
The dealer sits in the South seat and turns up the ace of spades. West passes as does North. East then orders the dealer to pick up the ace.
West starts off the hand by leading the 9 of spades (trump). North is void and throws off a 9 of clubs. The maker(E) plays the right. And the dealer(S) plays the card they picked up, the ace of spades, while still holding 2 lower spades in his hand. The bidding team has their first trick and 3 trump have been played.
Next the maker(E) leads the left. South follows suit with the 10 of spades. West and North both throw off small diamonds. They now have 2 tricks in with 5 trump played. The maker(E) now knows that South may hold the 7th trump. To avoid a euchre he has find a way to, either have South use that last trump, or have his partner take a trick.
He decides the best way would be to lead his best non-trump card.
Unfortunately he picked the wrong card. This gave 1st seat the opportunity for a euchre.
Now let's assume south had played the 10 on the makers right. The maker would have known South still held the ace. If then he had lead either off suits instead of the left for second trick, he would still hold queen - left. These cards would put him in a position to make his point.
Here's some other ways info is given out:
Many inexperienced players will give away the fact that they can follow suit. Say clubs are lead, and they only have one club in their hand. They will hold their hand on the card, waiting their turn to use it. This tells the opposition that they have a club and will not trump the lead. Now your opponents have a lot of information as to what card they should play.
Another thing to avoid is sorting your cards after picking the up-card. People tend to place all their trump on the same side. This could give an observant opponent information on how many trump are in your hand.
Suggested Further Reading: