What is “tension”?
I would say it is the product of my concentrated Attention to something.

It is the energy that my attention provides.

Depending on the allocation of my Attention, this “energy” or tension could be either fruitful or stressful.

What Frankl means ,when he writes, that there is a need to have a direction and then to provide the energy / tension through attention, it is the combination of consciousness and attention.

“Consciousness and attention appear as two closely linked systems, each controlling and being controlled by the other.

The first contains information and provides direction, the second provides energy and new information by introducing unplanned variation into consciousness”

(Mihaly Csikszentmihalyi Flow and the Foundations of Positive Psychology 2014, p.29)

What is then the difference between optimal and aversive states of concentration?

The only answer that fits appears to be a very simple one.

“Optimal experiences occur when a person voluntarily focuses his attention on a limited stimulus field, while aversive experiences involve involuntary focusing of attention. In other words, the individual’s choice determines the quality of the experience”

(Mihaly Csikszentmihalyi Flow and the Foundations of Positive Psychology 2014, p.31)

With the entry of consciousness into experience, a new principle has established itself.

Attention is focused upon one thing to the exclusion of the rest.

The mere fact that something is present to sense does not give it a claim to attention….

Consciousness is absolutely autonomous: it’s decision alone determines whether a given sensum or emotion shall be attended to or not.

A conscious being is not thereby free to decide what feeling he shall have;
but he is free to decide what feeling he shall place in the focus of his consciousness

(Collingwood 1938, p. 207)

Viewing a future of excellence according to one’s uniqueness can provide a direction in the present.

This attitude requires to make a healthy effort to build or develop a character.

That healthy effort provides meaning to one’s life, otherwise living is experienced as meaningless and one is in an existential void state.

Character building is much greater and more important than the building of a house, a city, a nation, or an empire.

One might ask why it is so important, as it is only the building of our insignificant self, while many have built an edifice or even a nation and they are gone and there is no memory of them left.

The Taj Mahal is one of the most wonderful buildings in the world. Those who see it – artists, architects – have a great admiration for it, but that is all.

No one cares who made it; no one’s heart is moved on account of the builder.

To this day the Hindus repeat, early in the morning, “Ram, Ram” the Buddhists call on the Lord Buddha and the Christians on Christ.

Why?

Only because of the personalities of these holy ones, the magnetism that was theirs. The words of Christ spoken so many hundreds of years ago are remembered today simply because of his personality.

The Art of Being and Becoming Hazrat Inayat Khan 2012, Omega Publications, p.210