What do you mean by "Trim"?

[RichLockyer]

Grajan once bubbled...
This is precisely why fin pivots are such a dreadful training aid. A well trimmed diver who breaths in should float off the bottom entirely - not just at one end.

Actually, fin pivots are not that bad.
A properly trimmed diver, when his legs are FULLY extended, will drop the feet and raise the head.
Likewise, a properly trimmed diver, pulling his knees to a fully bent position, will raise his legs and drop his head.

Weighting for any other situation renders the diver unable to obtain one or the other extreme.

It's all about balance, and you balance under water the same way you balance in the air, except that your center of rotation is slightly different. It's no different than balancing yourself on a board that is supported by a pivot (see-saw)... using only your legs and arms (but not actually finning), you should be able to "work" the see-saw.

 

[RichLockyer]

mgri once bubbled...
so let me see if I got this right. To obtain static horizontal trim you have adjust your weight distribution.

Once static horizontal trim is tuned in. Learn to change your trim through body mechanics.

Sounds easy enough on paper...

And not that difficult in the water IF the weight distribution is on the ball.
Rotationally, I can handle about a 2-pound off-balance on my left/right trim and not notice it. 3 pounds and I begin to roll and either have to extend one arm or fin harder with one leg.

How would I know?

It's how I balanced my canister light. Rather than replacing one of the 4 pound weights in my belt with the canister for night dives, I simply shifted 2 pounds from the right to the left. Adding the canister then gave me a more comfortable level of being slightly overweighted for shallow lobster hunting, and overbalanced the belt by the same 2 pounds.

Now that I've got the Helios pack, I just load the canister for all dives and keep the belt balanced.