Re: How do you practice liver blows?
Well, if you're winging wide hard shots at your sparring partner, then sure they're gonna know you're trying to hurt them. And that may not be entirely bad - like the others have suggested.
But like Virus said - it doesn't take a whole lot of power to hurt someone.
A proper liver shot is a short quick shot. No big windup or wide looping shot needed. It hits your partner fast and drops them in an instant. In other words, the most effective liver shot is a quick discreet sneaky little punch - if you deliver it right, it should LOOK as though you simply got lucky.
There are some good videos on youtube that demonstrate how to not only practice throwing one, but how pros have applied this in actual fights. Keeping it short and quick requires a little setup. In other words, it's rarely thrown as a single punch. It's usually on the tail end of a combo - where it is thrown quickly in short range.
Good luck practicing this.