Its MDC firing cord or a line shaped charge of C4 explosives. Its also called "det cord". Its designed to shatter the canopy prior to the seat and pilot shooting through it on ejection. The zig zag design is provided to ensure that the correct concussion from the explosion occurs to shatter the whole canopy completely. Its a milder explosion than typical C4 det cord I believe, someone correct me if I am wrong, so as to be safer to the ejecting pilots. I think the M stands for mild. Anyway, the T-45 has them as well. The MDC can also be fired manually for an Emergency Ground Egress, such as a fire on the ground or something and you can't get the canopy open. Sorry, I digress..