Complete concentration is one of the conditions that can make flow more likely. When your attention stays with a clear and appropriately challenging task, it becomes easier to become absorbed in the work and less reactive to distractions.
Complete concentration means directing your attention towards the task at hand. It is not about forcing yourself to focus indefinitely. It is about creating enough space for the immediate challenge to hold your attention.
That level of focus can support thoughtful work and may help create the conditions associated with flow: deep involvement, clear goals and a close match between the challenge and your skills.
Flow cannot be guaranteed or switched on to order. However, when a task is clear, feedback is available, distractions are limited and the challenge feels appropriate, sustained attention may make deep absorption more likely.
Notice which conditions support your best work, then design more of those conditions into your day. The aim is not uninterrupted focus at all times. It is the ability to protect your attention when the work matters.