A rotary egg beater is an egg beater that has rotary blades for beating, whipping, mixing, etc
The rotary egg beater is a hand held and hand crank operated whisk and beater to beat eggs and other ingredients together.
It's like an electric hand held mixer but is operated by hand crank.
An egg beater may also be an electric hand mixer as well.
It's a rotary tool with beater attachments that mixes eggs up and other ingredients as well.
You can beat eggs by hand or use an electric mixer to beat the eggs with.
You can also get beater attachments for electric stand mixers or even beat eggs with a whisk or a couple of forks although beating eggs by hand takes some arm strength.