When the motor is trimmed up the prop ends up being pointed down and pushes the stern of the boat down. This causes the bow to rise. So when the motor is trimmed up the nose goes up.
When the motor is trimmed down the prop ends up being pointed more even or up and pushes the stern up. This causes the bow to go down. So when the motor is trimmed down the nose goes down.
Trim is not a set it and forget it sort of thing. It may need adjusting frequently depending on conditions.
Finding proper trim is more of an art than a science. It will be different for each boat and even for each speed on that boat.
You want to achieve maximum efficiency (speed) with maximum smoothness of ride. If you trim too low you end up plowing through the water and in some cases even taking water over the bow. (Which is OK for washing the carpet but not a whole lot of good for anything else). If you trim too high you end up either porpoising as the bow is pushed up and then drops back down or you lose speed as the prop comes out of the water (RPMs will go way up).
Take the boat out on a calm day and set a straight course. Set your throttle to a steady RPM and play with the trim. Start about 1/4 up and slowly adjust it up and down. watch your GPS to find where your fastest speed is for that RPM. Remember that setting. Do this for a range of different RPMS and it will give you a starting point for your next trip out.
The trim setting will vary from day to day due to boat load, weather conditions, phase of the moon and whether the boat just feels like behaving or not.
You may also need to adjust trim in turns (I do on my boat as I lose grip a bit in very tight turns).
Just go out and have fun playing with it.
