u/#q2679

The link takes you to the NASA page that explains the countdown and what they do at each hour/minute milestone before launch, including holding (pausing) to check everything and continue the countdown. T-9 minutes and holding/counting is the last milestone before T-0, Liftoff!

T-9 minutes and holding:

This is the final built-in hold, and varies in length depending on the mission.

Final launch window determination

Activate flight recorders

Final "go/no-go" launch polls conducted by NASA Test Director, Mission

Management Team and launch director

T-9 minutes and counting:

Start automatic ground launch sequencer

Retract orbiter access arm (T-7 minutes, 30 seconds)

Start auxiliary power units (T-5 minutes, 0 seconds)

Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)

Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)

Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds)

Crew members close and lock their visors (T-2 minutes, 0 seconds)

Orbiter transfers from ground to internal power (T-50 seconds)

Ground launch sequencer is go for auto sequence start (T-31 seconds)

Activate launch pad sound suppression system (T-16 seconds)

Activate main engine hydrogen burnoff system (T-10 seconds)

Main engine start (T-6.6 seconds)

T-0

LIFTOFF!!!