LABS stands for Low Altitude Bombing System or Lob Bombing for delivering a nuclear weapon.
The A/c approached the target at low altitude and at a pre-determined point pulled up sharply. At a given point in the climb the bomb is released and is lobbed towards the target. The A/c continues to climb executes a roll out at the top of the climb and heads off back the way it came thereby escaping from the effects of the bomb.
I'm sure Pilots who carried out this manouvre will give you more details.