The specifics of what posture adjustments do to your trim will vary from person to person, and even from one equipment setup to another. If you are using Jets with spring straps, the chances are that your feet will be negative enough that extending your legs will tend to tip you feet down/head up. If you are wearing very heavy neoprene on your legs, and using neutral or positive fins, doing the same thing might make you more head down. We all have to play with posture to see what the effects are.
This is a fun thing to do during a dive with a good buddy: Tell your buddy beforehand that you want feedback on your trim. Get somewhere where you can get in a good horizontal position while swimming. Stop, and see how you tilt. Ask your buddy for feedback -- he can tell you if your body is flat from shoulders to knees, and which way you are tilting. Then change your posture -- bend your knees and see if anything changes. Extend your arms. Bring your head back to the tank valve, or tip it down. You can do a lot of useful research in just a few minutes.
As Rick says, posture is key. When I started diving doubles, I spent several months moving things around and changing plates and the like, only to discover that, within a fairly broad range, I can trim out anything with my body position.
This is a fun thing to do during a dive with a good buddy: Tell your buddy beforehand that you want feedback on your trim. Get somewhere where you can get in a good horizontal position while swimming. Stop, and see how you tilt. Ask your buddy for feedback -- he can tell you if your body is flat from shoulders to knees, and which way you are tilting. Then change your posture -- bend your knees and see if anything changes. Extend your arms. Bring your head back to the tank valve, or tip it down. You can do a lot of useful research in just a few minutes.
As Rick says, posture is key. When I started diving doubles, I spent several months moving things around and changing plates and the like, only to discover that, within a fairly broad range, I can trim out anything with my body position.