In SWINGING, the left arm is SWUNG across the chest in the backswing, then is FURTHER LOADED against the chest in start down and the early portion of the downstroke.
As the hips near a halt and then the shoulders, the left arm starts swinging BACK the "other way" —this is the #4 accumulator release— and although it is releas
ing at impact, it is STILL ACROSS THE CHEST.
It reaches its "in line" condition—90° to the shoulders—when both arm are straight in the FOLLOW-THROUGH. At that point the arms often, but not always, past the chest and the left arm starts bending.
The so-called "Blast-Off" is simply the release of the fully loaded left arm(most the arm arm is across the chest), to it's in-line condition.