On the Boeing 737 the trim wheels are directly connected to the stabilizer jackscrew with cables. The force required to move the trim wheels depends on the load on the stabilizer. The more you are out of trim, the more force is required to move the stabilizer.
From the 737 NG FCOM (9.20.8 Flight Controls - System Description - Stabilizer Trim, emphasis mine):
Manual stabilizer control is accomplished through cables which allow the pilot to
position the stabilizer by rotating the stabilizer trim wheels. The stabilizer is held
in position by two independent brake systems. Manual rotation of the trim wheels
can be used to override autopilot or main electric trim. The effort required to
manually rotate the stabilizer trim wheels may be higher under certain flight
conditions. Grasping the stabilizer trim wheel will stop stabilizer motion.
left: trim wheel with handle stowed (source), right: jackscrew in the tail (source)
The FCTM (8.17 Non-Normal Operations) explains the correct procedure for manual stabilizer trim (emphasis mine):
Hold the control column firmly to maintain the desired pitch attitude. If
uncommanded trim motion continues, the stabilizer trim commands are
interrupted when the control column is displaced in the opposite direction.
Manual Stabilizer Trim
If manual stabilizer trim is necessary, ensure both stabilizer trim cutout switches
are in CUTOUT prior to extending the manual trim wheel handles.
Excessive airloads on the stabilizer may require effort by both pilots to correct the
mis-trim. In extreme cases it may be necessary to aerodynamically relieve the
airloads to allow manual trimming. Accelerate or decelerate towards the in-trim
speed while attempting to trim manually.
Anticipate the trim changes required for the approach. Configure the airplane
early in the approach. When reaching the landing configuration, maintain as
constant a trim setting as possible. If a go-around is required, anticipate the trim
changes as airspeed increases.
Note: this FCTM is from a 737 NG, not a MAX. The part about "trim commands are interrupted when the control column is displaced in the opposite direction" is no longer true when MCAS is active on the MAX, the rest is still valid though.
You can also see this procedure applied in this YouTube video by Mentour Pilot.
Regarding the Ethiopian crash: I don't want to speculate on what the crew did, but if
the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units
then it did indeed move in the wrong (AND - Aircraft Nose Down) direction.