Difficulty concentrating is usually treated as a problem of discipline. More often it's a problem of state — you can't focus well while braced, depleted or holding several unresolved things at once, and no amount of trying harder changes that.
What actually interferes with focus
Mostly it's state, not willpower — fragmented attention, shallow breathing, held tension and fatigue all degrade concentration while going unnoticed by the person they're happening to. The stress-and-focus side of this is covered in "Brain Fog and Focus"; what follows is what a movement practice adds.
Why movement before thinking
Trying to concentrate from a scattered state doesn't work well. Changing the state first is often more effective than pushing against it.
A short movement practice does that physically — breathing slows, tension releases, attention narrows to the body because that's what the practice asks for. People commonly report that the quality of attention afterwards is different.
The attention training part
The less obvious benefit. Qigong requires sustained attention on something unexciting — a slow movement, a breath — which is the same capacity reading and concentrated work require.
Practising attention on something simple appears to make it more available for things that are harder. That's the claim meditation research makes too, and it's reasonably supported, though the size of the effect is debated.
When it's useful
Before concentrated work, as a deliberate transition rather than sliding from email into something difficult. In the mid-afternoon slump, where five minutes of movement usually beats more coffee. And after a day of meetings, to clear before starting again.
What it isn't
Not a treatment for attention disorders. If concentration difficulties have been lifelong and significantly affect your work or relationships, that's worth discussing with a doctor — there are specific assessments and treatments, and general wellness practice is not among them.
Not a substitute for sleep either. Nothing compensates for being short of it.
The practical version
Five to ten minutes is enough for the state change. Longer practice has other benefits but the focus effect doesn't require much.
Same time each day helps, because the routine itself starts signalling what comes next.