They look for a host then jump on their host's head then it becomes a part of the victim's nervous system. It most likely takes control of most of the motor cortex, the part of the brain controlling movement. This explains how the the host can still speak.
The exact process is unknown, but the crab does have a line of teeth, which Kliener calls 'beaks'. These beaks latch onto the head, but how it takes control is not elaborated. It could excreet a toxin of some type, or insert some kind of protrusion innto the brain.
The toxin that the headcrab might use may also be responsible for the hole in the hosts' chest and elongation of the bones in the hand to create "claws"