Great question about “grubbing” trout. I searched and came up with this answer. Interesting
http://www.thebookmailer.com/Gary/DynamicsNymph/DynamicsNymph2.html
'Grubbing is either a vacuuming or a rooting action. The fish holds at a downward slant, snout pressed into the rocks or vegetation. During vacuuming the trout flares its gills, pulling in water and insects, and then squeezes out the water. During rooting it actually scrapes nymphs off the rocks or eats them along with bits of vegetation.
"During rooting it actually scrapes nymphs off the rocks . . ."
Either way its head is down in the still water and its tail is up in the faster water, forcing it to constantly struggle to keep this position. The turning body causes the flashes of gold or silver, reflections the fisherman can see, that identify this form of feeding.