The idea is that bottom insulation is compressed by a person's body, rendering any loft in that area useless. Some modern down sleeping bags are through-baffled and under-filled such that the user can shift all the insulation to the top of their body thereby maximizing its potential to retain heat.
The idea behind this design is that bottom insulation gets compressed by a person's body, making any loft in that area ineffective. Modern down sleeping bags are designed with through-baffles and under-filling so users can move the insulation to the top of their body for maximum heat retention.