I saw your question and went to the rocketry forum for an answer. (www.rocketryforum.com) The answer is for APCP motors and not black powder motors. Aerotech and CTI manufacture their motors this way. The delay grain, for setting length of delay, is ignited when the motor ignites. The time of delay is calculated from the calculated moment of motor burn-out.

I hope this helps. BEAR

In other words, the motor and delay element both start burning at the same time. Blow thru often occurs when the delay element is not tightly sealed against the ignition source and then the powder lights pre-maturely. Sometimes, too powerful of an ignition spark comsumes/burns/combusts the delay element faster than desired and therefore shortens the overall performance. Sometimes, if the ignition spark is too far away from the delay element, that causes slower consumption and you get a bonus delay. I believe, even the more reliable delays in CTI motors sometimes have issues. Altimeter fired ejections are really the better way to go.