A blowtorch is used to heat the bottom of the membrane.
As the name implies, these are membranes that arrive at the site in the form of rolls. These are then unfurled and laid on a firm surface. The most common type of sheet based membrane is a bituminous waterproofing membrane. This type of membrane is stuck to the substrate with a hot tar based adhesive using blowtorches.
Joints between adjacent membranes are also made with the same hot adhesive. The sheets are overlapped by about 100mm (4") to form a waterproof joint. Some membranes are even joined by melting them with a hot air gun and then overlapping them on the previously laid sheet.
With this type of membrane, joints between sheets are critical, and must be done perfectly to avoid leakage.
Other types of sheet based membranes are PVC membranes and composite membranes. The latter have a fabric base that provides strength and tear resistance, and a chemical that coats the fabric to provide resistance.
Since these membranes are factory-produced excepting the joints, they are consistent in quality.