The product was designed to be used in cross matching safety fuses of the Bickford type.
This fuse is used in high-power model rocketry as a means of simultaneously igniting multiple "clustered" rocket motors.
A single flashbulb or flash pan is used to ignite pieces of Thermalite leading to each motor.
[1] Thermalite comes in three burn rates identifiable by the colour of the fuse wrapping: Thermalite igniter cords and connectors were manufactured in Quebec by ICA Canada Inc., however in November 1995 they ceased manufacturing igniter cord.
[2] Sources for thermalite are increasingly hard to come by and purchasing it by mail will usually require permits and licenses.