Here
are two documents that talk about and the use of Aerotech's
reloadable motor delay systems. Delay are used to time the ejection
charge to deploy the rocket's parachute. Timing needs to be close to
apogee. Otherwise if your parachute comes out early or late, you could
damage your rocket.
RMS delay for 29mm and 38mm motors - PDF delay document for 29mm and 38mm motors
RMS delay for 54mm motors - PDF delay document for
54mm motor