People trim according to the moon phases in order to acheive thickness of hair, or faster growing hair.
This was started (I think*) with farmers who began by planting and harvesting their crops according to the moon phases. The reasoning behind this is that the moon has an effect on the tides in the ocean, so it should have the same effect on crops and people which are also made up of about 60-80% water.
Here is one sites take on it:
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<font color="blue">One suitable explanation for the subtle effect is that during the phase when the moon increases in light (the waxing moon) fluids are drawn upwards as in growth. When the moon decreases in light (the waning moon) fluids flow to the roots. If you planted during a new moon, the growth energy available would increase the success and hardiness of a seedling. If, however, you planted after the full moon, energy for new seeds would decline. </font>
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So this same principal has been applied to hair growth. Not the actual "ends" but the roots of the hair are believed to be affected.
HTH /images/graemlins/smile.gif
*there may be other experts on the subject, but here's what I know.