he's got the head and arm ok and if you're looking at a Judo manual without instruction, you'd read that you do want as much leverage going into the choke as you can get by raising your hips - but here's the clincher - you always Always go perpendicular to the opponent as possible to prevent being rolled. It's a stability thing. Think about a table. A table with two legs is easy to tip, he's giving up a portion of his stability by initiating the choke. Sort of like in CMA - if you encounter chin na, you strike because he's engaged his hands into the lock. Well, in a head and arm, you've engaged your base into the lock - you have to keep your hips perpendicular so you can keep your center of mass far away from your opponent. In Judo, when you apply that lock / choke - you would put one knee into your opponent, the other (the near his head leg) is posted straight out with live toes. You try to circle with live toes to his head side. and again to reiterate, your knee on his leg side is in his ribs if you can manage.
You should also triangle your arms and imagine that you're popping his head off like you'd pop the head off of a dandelion. But this technique is actually classified as hold down or immobilization technique where you achieve ippon by holding your opponent, it's actually hard to choke out a person from this position if they have the cajones not to pu$$ out.