This is a trick from the old folks here that when a person is on a diet, they would advice him to use the other hand when eating. That is when you are right-handed you should use your left hand when you put food in your mouth and vice versa if you are left-handed. I haven’t tried it but I guess it has some sense to it because it is not comfortable to eat with the other hand and that may cause you to lose your appetite somehow.
This really has no application when living the one meal a day lifestyle. It might help when a person was on some kind of a low calorie diet and trying to eat less; but with our one meal, there is no reason not to totally enjoy that meal, each and every day. It is actually part of the plan that you enjoy your meal, and you should not do things that make enjoying the meal difficult.
Since you are eating only one meal, you can have whatever you want for that one meal, and it is not going to be a lot of calories for that day. Plus, if you are doing low carb/keto, then you can have all you ant of any foods that are low carb; so either way, it makes no sense to try to eat with a hand that you are uncomfortable with.
Maybe, you need to read a little more about how the OMAD diet works for weight loss, and this would make more sense to you ?