A favorite — but wasteful — means to control the direct-expansion loop is with a hot-gas bypass valve. A thermal-expansion valve maintains a constant temperature of the fluid in the discharge line. As a result, the system cannot modulate coil temperature when the return-air temperature varies, so that supply air is delivered at a constant temperature. The hot-gas bypass valve introduces hot gas into the fluid stream to modulate the temperature, so there can be better control over the supply-air temperature. This is the fluid side equivalent of reheat, though on a much greater scale as it occurs at all loading conditions, by design.
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Excerpt from Energy Conservation Projects.