Flow is a mental state in which a person is fully immersed in what he or she is doing, characterised by a feeling of energised focus. This is a concept authored by Mihaly Csikszentmihalyi.  

As Csikszentmihalyi says, flow has these characteristics:

1. Clear goals
2. Concentrating on one activity deeply
3. a lack of self-consciousness
4. distorted sense of time
5. immediate feedback about successes and failures
6. balance between ability level and challenge
7. A sense of personal control over the situation
8. Intrinsically rewarding to you

Not all characteristics must be present for there to be flow.

The other important thing is a non-disturbing environment. This is why I ask you to work in this room quietly. Voices are to be at whisper level so we do not disturb others!

Do not ignore the one crucial experience for which we have been wired: prayer to God. Thus we begin lessons with prayer.

Overlearning is important for flow and it is a perfect way to study. 

Examples:

Playing an instrument as part of a group

Reading a really good novel

Playing a video game