Sometimes known as "drippy bees" this pattern of behaviour helps young, wax secreting aged bees to keep warm when exposed for colony examination. It is exhibited at temperatures below 11° C in still air and up to 15° C in moving air.
There are several components to this behaviour, which is often misinterpreted as running.
The first of these is a general flow of bees towards the lower edge of the frame and in particular to the lowest point of that edge where they form a bulge.
The second is the formation of blobs of bees at the point of greatest bee density, the lump may drop off as a blob or just hang as if it is about to drop off.
The third part is the formation of vertical strings of bees, with or without small blobs of bees along the length or at the bottom of the "string". With some strains... Several parallel strings may form a curtain of bees hanging from the bottom bar of the frame.
The activity is shown most in Apis mellifera mellifera strains and least in Apis mellifera ligustica types... Beo Cooper suggested strong links between this behaviour and cappings quality which certainly match the observations.
It is a simple matter when examining bees, to discipline yourself to position frames above the open brood box so that any dripping bees fall into the box, rather than on your feet.