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Substituted halonium ions have a greater amount of positive charge (more blue color) on the carbon atom with the larger number of alkyl substituents. This means that nucleophiles will attack at this position, explaining the regiochemistry of the halohydrin reaction (the OH group ends up on the more substituted carbon atom). The easiest way to understand this is to think about the halonium ion as a resonance hybrid as shown. Since the holonium ion requires attack from the side opposite the X atom, anti addition stereochemistry is always observed, leading to trans products.