Jelqing, depending on which it is, has many variations. The Slow Squash Jelq traps blood in the penis at 70% to 90% erection blood volume (not erection level) to perform an expansion. Horse jelq holds as much volume as possible close to 100% erection blood volume, and pushed directly towards the glans, using both hands at the base of the penis.
Regular jelqs is fast pull of blood, between 50% of erected blood volume and 100%, from the base of the penis to the base of the glans in one motion. Faster movement at flaccid state, or 50% of the erected blood volume, in a cycle of 1 two 2 seconds, commonly dubbed as angiogenesis exercise, creates a rise and fall blood pressure between +1 inHg to +6inHg. Normal jelq takes between 4 to 6 seconds, sometimes as fast as 2 to 3 seconds.
Hope that clears up some info about jelqs.