Full "in" means the leg (the outdrive, the bit with the propeller attached) is fully withdrawn "in" or "down" if you like. "Out" means the opposite, as in extended out or raised up. Just the terminology. If your rams are extended too much and the leg/outdrive is locked, or "jammed " in the up/out position, having someone pull down hard or even stand on the leg while you operate the "down/in" switch, can usually free it. Otherwise you may have to undo the rams to free the outdrive/leg.
I adjusted my trim sender by removing it from the leg. (Turning the steering hard to the appropriate side can give better access to the 2 fixing screws). Then, with my good woman on board, watching the gauge, with the leg in the full down position, I rotated the small knob at the back of the sender until the needle read "Down". I tested the gauge by slowly rotating the knob in the opposite direction, causing the gauge to read "Up". Rotated the knob back to the "Down" position, as shown on the gauge and fixed the sender back onto the leg. I fixed it on so that the screws are about midway along the fixing slots, giving room to tweak if needed. Don't rotate the sender once it is back on the leg or you'll throw it out again, Fix it straight on and tighten the screws.
Then raise and lower the leg and check that the gauge is corresponding. That's what worked for me.