Trim tabs work exactly the same way as the control surfaces on an airplane. As you know, there are three axes affecting the motion of your boat as you travel through the water: Yaw, Pitch and Roll.
Trim tabs have little effect on the yaw axis, because yaw is controlled by the rudder or the side to side movement of the outdrive. Where they are effective is in the roll axis, to eliminate or reduce listing or heeling, especially if your boat has a deep "V" hull, and in the pitch axis, to raise or lower your bow.
The port and starboard trim tabs act independently. It is a simple thing to adjust them to lift the down side of the boat and compensate for the three or four hundred kilos of weight caused by your passengers all wanting to be on the same side. To do this, lower the down side tab. 
The water pressing against the tab as you move will lift that side of the boat (around the roll axis) and eliminate your list. As your passengers move about, you can continue to adjust the tabs to compensate for the redistribution of weight. You must understand though, the trim tabs will have less effect at slower speed than at high speed. You can understand this by a simple experiment. Next time you are in your car put your hand out of the window at an angle to the slipstream. Note that when you drive slowly, the wind has little effect on your hand but as you speed up, the wind will eventually drive your hand up and back. Now remember, water is much more dense than air so prudence in applying trim is recommended.
The water pressing against the tab as you move will lift that side of the boat (around the roll axis) and eliminate your list. As your passengers move about, you can continue to adjust the tabs to compensate for the redistribution of weight. You must understand though, the trim tabs will have less effect at slower speed than at high speed. You can understand this by a simple experiment. Next time you are in your car put your hand out of the window at an angle to the slipstream. Note that when you drive slowly, the wind has little effect on your hand but as you speed up, the wind will eventually drive your hand up and back. Now remember, water is much more dense than air so prudence in applying trim is recommended.
Where most operators initially use trim is in the pitch axis, getting their boats to the plane mode as quickly as possible. It is then easy to maintain the boat’s most economical cruising speed by tab adjustment. This is accomplished using the "BowDown" control. This lowers both tabs and the force of the water against them will push the stern up consequently lowering the bow (around the pitch axis). Again, smooth rather than aggressive application is recommended to prevent pitch down of the bow. Trim may also be used in the pitch axis to keep the bow up to avoid taking seas over the bow if the water is rough. Naturally you would use the "Bow Up" control to do this.
I mentioned that trim tabs have little effect in the yaw axis, but they do have some. I’m sure you can readily understand that if you have only one of the tabs hanging out, besides lifting that side of the boat it will also cause a certain amount of drag. This will have the effect of slowing that side of the boat causing the bow to rotate about the yaw axis and turning the boat to that side. The rudder or out-drive are usually adjusted slightly to compensate.
Using trim tabs is much like riding a bicycle. You learn to do it by feel. Your knot meter will tell you when you have trimmed for best speed at any throttle setting and your common sense will help you adjust trim to sea conditions and weight distribution. I am positive that in gaining experience you will soon amaze your friends with your skills in boat handling and trim.
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