When calling a block, there is only one thing you know for sure: you have no way to stop a successful lone call. You don't know if they even have a lone.
You have no way of knowing where the trump are, they may even be in your partner's hand. While you don't expect to make your bid, it's nice if you can. If your partner was to have the boss cards, a first lead of trump could benefit by putting your partner in position to make a point. Give your team every possible opportunity.
The dealer is in the West position and turned up the jack of hearts. This is the second game we have played against these opponents. They won the last game with a successful lone call at 6 - 6. My partner(N), fearing another lone, blocks by ordering up the jack. I know some of you may question the wisdom of blocking at such a low score [2-1 in our favor]. However after looking at my partner's hand, I can see why.
Unfortunately, my partner just assumed that we would be euchred and led the 9 of spades instead of trump. The dealer(W) took this trick with the ace.
The dealer(W) then led back the ace of diamonds. My partner(N) took the trick with the ace of trump.
Then my partner(N) led back the king of spades. East seat played the king of trump and I (S) over trumped with the left. This left West seat with the boss cards. There was no longer anything I could do to save this hand.
Now go back and look at what would have happened had my partner (N) had led the ace of trump. It would have taken out East seats king and along with West seats right. I would have played the nine. This would have left me with the two boss trump. My partner had the boss king to pick up my losing ten of clubs. We would have made a point. We did go on to win at 9-9. However, this miss-play could have cost us the game.