I have searched this site and the web, and I don’t understand what to do after split bottom mount.
I try the underpass with my non-throw hand I end up in brain twister. ???
Please to help, thanks.
After Split bottom mount, bring your non- throw hand away from you and under the yoyo and put the yoyo on the string directly in front, then bring your throw hand back towards you and put the yoyo on the string to the left and then just swing it out. 
I thin you’re doing it correctly. After the rolls, slip out your finger and you should be in a Braintwister.
So, yeah you’re doing it right.
after the split bottom mount and the initial underpass:
• KEEP your throw hand index IN the loop.
• bring your two index fingers together.
• bring your throwhand index DOWN behind the yo-yo for the second underpass (again IN the loop the whole time). remember in the 2nd underpass you do NOT underpass the string segment connected to your finger. ONLY the loop over your throwhand index
• when it goes under the yo-yo and about halfway up, bring your throwhand index to the BACK of the loop (which is more a complicated loop than braintwister - it’s kind of a doubled loop, and unlike braintwister, BOTH index fingers are IN the loop)… and do the somersaults.
• dismount
• party