Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 30/03/2016
The second option I use for transitioning to mount from side control is the diagonal slide, which has a lot less steps compared to the reverse scarf hold approach. Start in side control, with a heavy cross face and your other arm under their far arm. Walk your other arm up the mat, until you can get their far arm tight to their head. At this point, you can grab just below their elbow with your cross facing hand, locking their arm to their skull (if you want extra control, you can use your head).
Drive your knee as high as you can on their body, sliding it diagonally over their chest. Aim to put your knee by their elbow. If you go too low, they may be able to snatch half guard. To prevent that, you can also use your free arm to block their legs, either simply shielding the area, or grabbing their near leg and pushing it away. The near leg is the one that will be on top if they try to drag your leg into half guard. If you have control of that top leg, it's impossible for them to get half guard: with just their bottom leg, they can't do much.
Teaching Notes: Locking that arm to their head seems to be important, as that messes up their attempts to escapes during the transition. In one of the older Grappler's Guide video, Jason Scully uses this version too, although he does it a bit differently, with more use of his head: that's something I could try using more. I was also finding you could switch right into armbars off the transition, possibly kimuras too, especially if you were able to get them to reach up over your shoulder.
Some people didn't want to 'spider walk' their fingers, instead doing something that looked a bit like the 'butterfly stroke' from swimming. Whatever works, I guess, so that's one I'll keep in mind for next time. ;)