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http://www.apollosaturn.com/asnr/docking.htm
Docking
The docking subsystem provides the means to connect and disconnect the lunar module and the command module. It is used twice during a normal lunar mission: at the beginning of the translunar flight when the CM docks with the LM, and in lunar orbit when the ascent stage of the lunar module docks with the CM.
The Docking Subsystem
Docking is achieved by maneuvering one of the modules close enough to the other so that a probe on the CM engages a drogue on the LM. In the first docking the CM is the maneuvering vehicle and the LM the passive one; in the second docking the LM ascent stage is active and the CM passive.
The probe, drogue, tension tie and a docking ring are the principal components of the docking subsystem. Each module also contains a docking pressure hatch and a tunnel through which the astronauts will transfer from one vehicle to another.
The docking maneuvers are controlled by the commander through short bursts of the reaction control engines on the active vehicle. He is aided in maneuvering his craft by the crewman alignment sight, an optical device something like the range finder of a camera which is mounted at a rendezvous window.
Before the docking maneuvers begin a crewman in the CM activates a switch which extends the probe. When the probe comes into contact with the drogue, it is guided into the socket at the bottom of the drogue. Three capture latches in the probe head then hold the two modules together.
A crewman then activates the probe retraction device (a nitrogen pressure system located in the probe) which automatically pulls the LM and CM together. At contact 12 latches mounted on the CM docking ring are automatically activated to form a pressure-tight seal between the two modules.
A CM crewman then equalizes the pressure between the LM and CM tunnels through a valve provided for the purpose and removes the CM's docking hatch. After removing the hatch, he first checks all the docking latches to make sure they are all engaged (locking any 3 of the 12 latches is sufficient to assure a pressure-tight seal and allow entry into the tunnel), and manually locks those that aren't. He then connects the two electrical umbilicals (stowed in the LM) to the CM ring connectors providing power from the CM to the LM. The probe and drogue are then removed and passed down into the CM along with the hatch. Finally, he operates the valve in the LM docking hatch to equalize the pressure in the LM. The LM hatch can then be opened; this is hinged to swing inward into the LM crew compartment.
In the first docking, the probe and drogue will not be removed, nor will the LM hatch be opened. the CM crewman will remove the CM hatch, check the locking latches, connect the umbilicals, and replace the CM hatch.
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