using eloquent interfaces pledge programming opposing an interface

i have seen arguments controlling eloquent interfaces slight locking classes use interface. justification seems forcing others way interface pledge improved decoupling classes grant easier testing.

example:

public interface icut
{
    vacant cut();
}
public category blade : icut
{
    vacant icut.cut()
    {
        //cut something
    }
}

and blade object:

icut obj = new knife();
obj.cut();

would advise slight interface implementation? since why not?

edit:
also, given i am controlling an eloquent interface following work.

knife obj = new knife();
obj.cut();