It is basically a charge similar to a 105 atry round that shoots your ass out of the disabled aircraft. From my short story on this subject:
Tracer rounds drifted up towards us, flashing from the big guns below. Initially, the few red-orange balls floating up – five at a time, desultorily - seemed harmless; even eerily beautiful. “When my eyes were stabbed by the flash of a neon light that split the night, and touched the sound of silence,” played the sentient masterpiece of Paul and Art. Then, as suddenly as a desert cloudburst, the anti-aircraft fire poured up in sheets. The NVA gunners were shooting payback from three guns; payback for the B-52 strikes. Payback, as the saying goes, is a motherfucker.
Twenty-four terrified and pissed-off young North Vietnamese soldiers, some probably chained to the guns, were shooting at us with inch-and-a-half explosive tracer shells, fed in five-round charger clips, with a rate-of-fire of 180 rounds-per-minute. The 37 mike-mike anti-aircraft crews tracked our Grumman Mohawk, bracketing us with thunder and lightning. “But my words like silent raindrops fell, and echoed in the wells of silence,” the folk singers’ haunting lyrics addressed my darkest fears.
I pushed the nose over and dived for a lower altitude, out of the 37 mike-mike’s kill zone. “Look!” screamed Charlie. “Starboard, low!”
A stream of green tracers, like water from a fire hose, arched up from the side of a hill just to our right. A 14.5-millimeter Soviet-built ZPU, firing 600 rounds-per-minute of explosive ammunition, was shooting at close range; way too close. I yanked the Mohawk into a tight, high-G left turn to escape the ZPU emplacement, only to have another ZPU – a quad-barreled ZPU-4 – open up from my port side.
“Flak trap! Flak trap!” I shouted, redundantly, at the now terrified Charlie Walker. A round ripped through the Mohawk’s flak curtain and canopy, sending ballistic-proof glass shards into the night void. Another round slammed into the trailing edge of the starboard wing, exploding. The aircraft shuttered violently as a round hit the tail. I fought for control and dived for the relatively safety of the tree tops.
“MAYDAY! MAYDAY! MAYDAY!” I managed to yell on the radio. “Crazy Cat 9-1 is hit. Just south of Tchepone. Flak trap.”