Stills

I think that little touch of color from the rose makes the rest of this pop.

The bit of motion blur in the guitarist's hands also work well.

I'm normally not a fan of cutting off the top of someone's head, but in this case it serves to highlight his expression.

Yep, this works on a lot points.
 

I'm not sure what the button on the wall top right is but it serves to hold everything else in the frame.

The red works. The motion is good.

Although you've titled this 'concentration' for me the subject is the left hand and the strings. The face, whilst necessary, isn't sufficient. My eye goes to the centre and the four boundary points (flowers, button, face and light guitar wood) keep it there.
 

This doesn't work very well for me: it's partly composition, partly background, partly the combination of clothes and their colours (some of which are very subjective oservations). The composition looks (more importantly, feels) as though it has been skewed to include the roses (even if that wasn't the intent) although I have no problem per se with the cut off head and hand. The blurred left hand is neither one thing nor another even if the vibrating string does convey the playing: the pinkness of the blurred fingers is part of the colour difficulties I have overall, the roses being the prime culprit. They are in the wrong plane: too blurred to show us a rose, not blurred enough to become background. Nor is the fact that their pot is in a white plastic bag helping - are the roses his or are they a gift to a busker? Is this taken in winter (his mittens tend to indicate that)? If so what is a fully flowering rose doing here?

I've tried standing back and being more objective but the overall colour clash and the peculiarity of the context just keep jarring.
 

Concentration suits this one Rex. I sometimes get that look on my face when playing, usually due to the pain of the strings and the fear of fluffing that next chord change. The motion in the fingers works well as does the tightness of the crop. The roses fill that space well.
 

John Ellis pretty much matched my thoughts on this too. As a study of concentration the face works but the other elements of the shot conspire against an enjoyable complete work.