He wanted to know why I roll over the opponent when she is in this position.
There was a game we used to do when I was a child which illustrated this perfectly. It is also strongly discouraged by all of the judo organizations for the very logical reason that it is dangerous. It looks dangerous, although I have never known anyone to get injured doing it. The same can be said of climbing trees, diving off a pier, swimming at Johnson's Shut-Ins, and swinging on a rope swing and letting go to land in the middle of a river with a strong current, barely making it back to shore and starting all over again. All of these are things I had fun doing as a child but other people died doing them.
Here is a video from Fox Valley Judo showing adults diving over a crash pad. This is kind of the motion you are going for. It's somewhat safer than the drill we used to do because the person is landing on a crash pad. Here is another video that has players diving over a standing person and then over some mats. In this one, too, they land on the crash pad. Both of these have the player diving up more than over.
The drill we used to do is not shown anywhere that I can find because it is stupid and dangerous, no doubt. What we did was have a person bent down like Ana is in the picture above, and then we would do a forward roll over them. After the whole line of kids did that, we'd have a second person kneel next to them and do a rolling fall over both people. If you chickened out, or if you missed and ended up falling on the second person, you were out. We kept adding people until only one person was left.
I still did this drill with my own children. They can't sue me because if the do, I'll put arsenic in their toast and that will be the end of that.
ANYWAY .... the point is that you do a DIVING break fall , stretching your body out and then rolling back like a regular fall. Your MOMENTUM carries you over
A second important point is that even with the momentum, if your opponent is MUCH heavier, this will not work. Say I am 123 pounds (56kg) and my opponent is 220 pounds (100kg). I'll probably get him somewhat off balance but I doubt I will pull him all the way over.
This technique works fine for competition because it is by weight division, and against a person of equal weight, with the momentum added by the diving motion, I can always pull them over and up.
As you can see from this helpful wikipedia entry (give them money, I did),
Kinetic energy = 1/2 mass times velocity squared
This is why the diving part is so important. The more I can increase my velocity by diving out, since this value is SQUARED, the greater impact on total force applied to pull my opponent over.