The white stuff, more than likely, was a sperm plug or plugs if your snake is a male and old enough to produce them.
As to the amount of pressure that would damage a snake, I couldn't say. Someone with more experience than me would have to answer that. Best bet is to find an experienced breeder or herper in your area that could demonstrate proper technique for popping and help to show you hands-on how to do it correctly, for you and your snake both. YouTube has some decent videos showing the technique as well, but nothing beats being shown in person.
Good luck!