One of the chief benefits of practicing jelq and other "enlargement" techniques is that it induces the penile tissues to become "extra engorged" (swollen) with more blood and lymph fluids. The idea is to allow those fluids to remain within the inner chambers of the penile shaft which, over a (very long!) period of time will gradually make those chambers get bigger and hold more fluids (hence a bigger size increase--again over time of course).
When you ejaculate immediately after jelq or other exercise, you risk losing the optimum benefits (obviously) as your penis tissues immediately relax.
Your member may still look "puffy" from the exercise regimen, but the chances of keeping those extra amounts of blood within the inner chambers for an extended duration---are significantly reduced. It's best then, to avoid ejaculating.
Eventually, after doing PE awhile, many men will learn to simply ignore those urges to "finish up" by masturbating.
But even THAT takes time to achieve, too.