Explosive core

Detonating cord (also called detonation cord, detacord, det. cord, detcord, primer cord or sun cord) is a thin, flexible plastic tube usually filled with pentaerythritol tetranitrate (PETN, pentrite). With the PETN exploding at a rate of approximately 6400 m/s, any common length of detonation cord appears to explode instantaneously. It is a high-speed fuse which explodes, rather than burns, and is suitable for detonating high explosives. The velocity of detonation is sufficient to use it for synchronising multiple charges to detonate almost simultaneously even if the charges are placed at different distances from the point of initiation. It is used to reliably and inexpensively chain together multiple explosive charges. Typical uses include mining, drilling, demolitions, and warfare.

"Cordtex" and "Primacord" are two of many trademarks which have slipped into use as a generic term for this material.

As a transmission medium, it can act as a downline between the initiator (usually a trigger) and the blast area, and as a trunkline connecting several different explosive charges. As a timing mechanism, detonation cord detonates at a very reliable rate (about 7,000–8,000 m/s), enabling engineers to control the pattern in which charges are detonated. This is particularly useful for demolitions, when structural elements need to be destroyed in a specific order to control the collapse of a building.

While it looks like nylon cord, the core is a compressed powdered explosive, usually PETN (pentrite), and it is initiated by the use of a blasting cap. Detonation cord will initiate most commercial high explosives (dynamite, gelignite, sensitised gels, etc.) but will not initiate less sensitive blasting agents like ANFO on its own. 25 to 50 grain/foot (5.3 to 10.6 g/m) detonation cord has approximately the same initiating power as a #8 blasting cap in every 2 to 4 inches (5 to 10 cm) along its entire length. A small charge of PETN, TNT, or other explosive booster is required to bridge between the cord and a charge of insensitive blasting agent like ANFO or most water gels.

...
Wikipedia