How can an emergency blanket keep an immobile person warm

Easy. It helps to minimize loss of body heat.

Light (and heat) is absorbed by dark colours and reflected by light (or silvery) colours.
The body heat is reflected back to the patient and outside warmth is drawn to the person.

It keeps the body from air flow which reduces body heat. In other words, it prevents evaporation.

The blankets work by two methods-
- they reflect heat back to the victim slowing heat loss
- they block the wind and trap a layer of warm air as insulation against the body.

- David