you are most likely causing retrograde ejaculation. in other words the semen is going into your urinary bladder instead of out of your penis. i do not recommend that you do this because it can create problems at a later time when you are trying to conceive. there is no harm with masturbating but do not do this "squeeze" technique any more.
weird mastrubating problem. i have been doing this for the last 4 years! . this may sound a bit weird but i have a bad habit of holding my sperms inside by squeezing my penis hard enough so that it doesn't come out. this first happened by mistake but later it seems i don't like cleaning up my ejaculation so i just do this again and again. i don't think this is safe or healthy. would you elaborate what kind of problems this can cause? also what happens to the sperm that doesn't come out? and if any damage has been caused till date how can i know & possibly cure it.