Reflexes work because sensory neurons bring sensory info almost directly to motor neurons in a specific area of the spinal cord. Because of this, the body can react to certain stimuli even before the brain registers what is happening.
The quickest reflexes are called monosynaptic reflexes because only one synapse is involved in the spinal cord, allowing almost no separation between the perception of sensory information and the motor neuron reflex. This is the reflex involved in the classic test where a doctor taps a person on the knee and the knee involuntarily kicks out. This occurs because monosynaptic reflexes sense and correct any unexplained changes in the body's movement. Monosynaptic reflexes also explain how an individual is able to walk and conduct many motions without having to put much thought into it.
Disynaptic reflexes involve at least one extra neuron between the sensory and motor neurons, while polysynaptic reflexes involve the spinal cord having more than two synapses. In all reflexes, it takes more time for the information to go from the spinal cord to the brain than from the spinal cord to the affected part of the body, which is why the body feels sensations such as the pain from a hot stove and reacts even before the mind has processed exactly what is happening.