I have never had my cock go limp while pumping. The vacuum just about makes it impossible. I don’t like to enter the tube totally flaccid because it’s easier to get your balls sucked in and too much fat pad skin gets involved. I don’t like to enter the tube totally erect either, because then your tunica is rigid and therefore not flexible enough to expand. I like to enter the tube around 80-90%. Then everything is flexible enough to take on maximum expansion as provided by the vacuum.
The vacuum allows for expansion in all directions including length, but girth is most greatly effected and you will probably gain anywhere from 1/4 to 1/2 inch in girth after a good pump session. Your head will also take on new proportions never before experienced.
The key is then to repeat this process so the temporary gains become permanent. It seems to be best to combine jelqing with pumping. I start and finish my routines with girth blasters, and after every 10 minutes of pumping, do 5 minutes of girth blasters. So it’s jelq, pump, jelq, pump, jelq. I do either 3 or 4 pump cycles like this depending on time availability or level of cock fatigue.
Now I think it’s best you begin your pumping sessions before asking more questions, because you will never remember all of this without being able to do it.