The two main enzymes active during the mash are alpha and beta amylase. Alpha amylase, which is most active in the 154-167F/68-75C range, creates longer sugar chains that are less fermentable, resulting in a beer with more body. Beta amylase, which is most active between 130-150F/54-65 C trims off single maltose sugar units that are more fermentable. This results in a more complete fermentation (higher attenuation) and a cleaner beer with a thinner body.
A low step temperature (146-150F/63-66 C) emphasizing beta amylase will therefore result in a more complete conversion to simple sugars, but will take longer to complete. These simple sugars will ferment more readily, producing a highly attenuated beer that has higher alcohol content but less body and mouth-feel.
Conversely, a high temperature conversion step (154F-156F/68-69 C) emphasizing alpha amylase gives you more unfermentable sugars, resulting in lower alcohol content and a full bodied beer with a lot of mouth-feel. Moderate conversion temperatures (150-153F/65-67C) result in a medium body beer. In BeerSmith the mash profiles are labeled light, medium and full bodied to make this selection easy.
Conversion time also varies with temperature. Complete conversion of your malt for a low temperature, light bodied profile takes longer than a high temperature, full bodied mash profile.
Starch gelatinization ranges: