A thermobaric weapon (or solid fuel-air explosive) uses the gaseous products (H2, H2O, CO and CO2) of an initial explosion for an afterburning of reactive solids. Because their reaction with atmospheric oxygen only produces solid oxides the blast wave is primarily generated by heat of combustion ("thermobaric") instead of expanding explosion gases. This makes thermobaric explosives more effective in oxygen deficient environments such as tunnels, caves or underground bunkers. Rather than providing protection as they would from conventional explosive ammunition, structure interior walls, particularly cement or other hard surfaces, magnify and channel the shockwaves created by a thermobaric detonation. The stronger the walls, the higher the pressure’s reflective effect.
You can read about the mechanism on wikipedia
http://en.wikipedia.org/wiki/Thermobaric_weapon