S Mount Problems

[Julian]

http://www.10thplanetjj.com/techniqu...-option-mount/hey guys been trying out the option from above and I'm having problems with the s mount, i never seem to quite get it, I'm not too sure about the space I'm creating or where my weight should be when I bring my leg round, sud my ass be on his chest? what keeps him from exploding out when I swing my leg around to make the S before I go for the armbar?

any help would be appreciated, thanks