This loner has all the highest cards of the trump suit, including the left and right bowers (both red jacks). No matter what card the opponents play, or who is leading the first trick, every trick will be taken by a player holding this hand.
Let's take a look at this hand:
I had an off-suit ace (the spade), and a handful of diamonds (the jack of hearts counts as a diamond if diamonds are trump). Importantly, I didn't have the jack of diamonds (right bower)! Without that, I was not sure to win all the tricks. However, it was just too tempting, so I called a loner.
This replayer shows how my loner went. If you prefer, you can view how the hand played out in the YouTube video below, but the replayer will show you all the cards in everyone's hands.
It turned out that the right bower was in my partner's hand, something you can see if you use the replayer above. This is how I won. At the worst, I might have only won a single point.If you view the replayer, you will see that my loner might have been foiled if the opponent to my right had led with a club. I might have trumped with my 9 of diamonds, and been over-trumped by the 10 of diamonds in my left-hand opponent's hand. So my right-hand opponent would probably have been wiser to lead the trick with a long suit (clubs, here), which sometimes works to stop a loner.
You are going to find disagreement about when it is worth going alone. Some people think it is rude to go alone with any but the best of hands. Since Euchre is a team game, they think that by going alone, you are throwing away the chance to win two points with the help of your teammate.
Other people are at the other end of the risk spectrum, and will call even risky loners that might get them set, losing two points.
I think most people recognize the value of a pretty good hand, and will be happy if their partner goes alone when the situation calls for it. I also think that one of the most important factors in playing Euchre is that the two teammates agree on strategy. Otherwise, it's not much fun. Teammates will start to bicker over whether this or that hand was a suitable loner, and the partnership will become acrimonious. Hopefully, everyone understands that both partners are trying to win the game, and doing it according to their best judgement!