This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From Fred Schecter:
A long lug (2 inches at least) or two smaller pieces of lug glued to
along standoff (the standoff is long in length, but does not stand off
from the rocket very far).
The standoff has bent/formed music wire epoxied to the top and
bottom. The bottom piece forms a little "U" shape that captures the
lower lip of the rocket body tube. The top piece is a flattened "V"
(almost a point) and it is angled slightly toward the rocket. This top
piece is inserted through a tiny hole in the body tube.
The entire rocket can sit on the lug while on the rod (that's why you
have the lower "U"). You apply a few wraps of masking tape to the top
of the launch rod and when the rocket takes off, it slides up the rod
until the lug hits the tape, The lug stops ("POPS OFF") but the rocket
keeps going straight since it has built up enough speed to fly stably.
The top pin MUST FIT SNUG!!!!! Otherwise it will pop off at
ignition. That would be bad.