According to the TNG: Technical Manual, when a "Yellow Alert" is called, a considerable number of things happen.
During Yellow Alert condition, all active bridge stations are
automatically brought to Full Enable Mode. Auto diagnostics (Level 4)
are initiated for all primary and tactical systems Ops is responsible
for evaluating all current operations and shipboard activities and
suspending any that may interfere with ship's readiness to respond to
potential crisis situations
Yellow and Red Alert protocols call for cross-trained personnel with
noncritical primary assignments to be available for their secondary
assignments as necessary.
Yellow and Red Alert operating rules generally require the presence of
at least two command personnel, in addition to Conn and Ops.
All on-duty crew and attached personnel are informed of the potential crisis via panel display and are directed to prepare for
possible emergency action. Second shift crew personnel are also
alerted and those in key operational positions are directed to prepare
for possible duty on five minutes' notice. Cross-trained second shift
personnel are directed to prepare for possible duty in their secondary
assignments. Specific systems preparations include:
Level 4 automated diagnostic series run on all ship's primary and tactical systems to determine ship's current readiness status.
If presently off-line, warp power core brought to full operating condition and maintained at 20% power output. Level 4 diagnostics
provide a status report on warp capability including maximum available
Main impulse propulsion system brought to full operating condition. At least one backup reactor element is brought to hot
standby. In Yellow Alert status triggered by potential hostile action,
Saucer Module impulse propulsion system is brought to partial standby.
All tactical and long-range sensor arrays are brought to full operational status. Secondary mission use of any sensor elements can
be overridden if required by bridge.
Deflector systems brought to full standby. Secondary deflector generators brought to partial standby. All operational backup
generators are energized to partial readiness.
Phaser banks are energized to partial standby. Power conduits are enabled, and targeting scanners are activated. Level 4 automated
diagnostics verify operational status.
Photon torpedo launchers are brought to partial stand-by. One torpedo device is energized to partial launch readiness and primed
with a standard antimatter charge, unless specifically overridden by
Ops or Tactical. Level 4 automated diagnostics confirm operational
The Battle Bridge is brought to partial standby status and backup bridge crews are notified for possible duty in the event of possible
Saucer sep maneuvers.
Two of the three shuttlebays are brought to launch readiness. The number of shuttlecraft at launch readiness is maintained at one.
Onboard sensors record the location of all personnel and alert Security of any anomalous activity. Location and activity information
is recorded for postmission analysis.
Level 5 automated diagnostics are performed to verify readiness of autonomous survival and recovery vehicle systems (lifeboats).
Note that although secondary deflector systems (e.g. the "shields") are brought to readiness, calling a Yellow Alert doesn't automatically engage the shields at full power, an action that might seem, to an enemy, to be a provocative action.