It's made to drive old solid grease out of stopped up fittings so that you can grease them. Fill with grease, then hit with hammer to drive grease through the fitting.
Yep that's what it is... Don't even think about using it on a zerk that is anything other that straight in. Use it on an angle zerk and you'll be trying to figure out what size EZ-out bit to use on the threads...
These were great to use on trucks with straight axles with king pins that wouldn't take grease.. I haven't seen one in maybe 30 years and it was probably 10 years before that when I last used it..