Barbs don't naturally swim in a weird position with their head down and tail up. It is normally caused by too much dry food. The fish gorge themselves on flakes and take in air at the same time. This causes them to become unbalanced and they swim with their head facing downwards.
The only other time they swim with their head down is if they are grubbing about in the gravel. But they should swim normally when they are no longer looking for food in the substrate.
They dont "swim" head down, almost pause like it for a brief moment and then carry on as normal. 5 of my 8 tiger(albino) barbs show this behaviour occasionally and they only get dry food once a week, the rest of the time they get frozen
Couple of my tiger barbs have always done this and show no other signs of ill health, always had me puzzled but i've pretty much just accepted it after several months of research!
im not sure about other barbs but im sure i read somewhere that tiger barbs headstand as a sign of submision to a fish higher up in the pecking order. some of my new tiger barbs do it to the alpha male in the school
That kinda fits with what occurs in my tank