A favorite food of Hooded Mergansers is crayfish (aka crawfish or crawdad). The birds swallow these small crustaceans whole, but they must first remove the two pincers, as they present a hazard. The technique for pincer removal is to grab the appendage near the base, and then shake and thrash it about until it breaks loose, as illustrated below.
The photo below depicts a male Hooded Merganser after emerging from beneath the water's surface with a fresh crayfish (with pincers intact):
At this point in the action (below), one pincer has already been removed (note the rear "stump"). The merganser has grabbed hold of the other pincer at its base:
With the help of its serrated bill, the merganser proceeds to shake the crayfish in an attempt to shear off the pincer:
Note below that the shaking has paid dividends, as the pincer has broken free from the crayfish:
The pincer falls into the water; it is of no interest to the merganser now. He has his eyes on a larger prize:
From my observations, it appears that the mergansers often dunk the crayfish in the water before swallowing it, possibly to remove any foreign debris:
This clean and pincer-free crayfish is now ready to be swallowed: