Literally, manita de puerco means "pork little hand." Hacer manita de puerco has nothing to do with preparing pork feet (see salir por patas). Hacer manita de puerco means "to twist somebody's arm in a painful way." It is a metaphor to indicate that the person whose arm is twisted is being forced to do something against her will.
The cartoon below shows a union leader haciéndole manita de puerco to a politician.
Cartoon "Manita de puerco" by Helguera.