I think I know exactly what this is. (My LO did this up until 9 months of age, then disappeared) It is called rhythmic movement disorder or possible even infant myclonic jerks. The first typically occurs only in the upper body, the ladder could be the trunk as well. It truly is not a disorder though in my opinion.
Think of a time when you are falling asleep and your whole body jerks you awake. Almost as if you thought you were falling. This is called a myclonic jerk. This is what your LO is doing. "AS LONG AS" when you try to alert LO to open their eyes it stops immediately. My PED and I went over this in length because I was worried it was a seizure. The first time it happened I rushed him there. They said #1- you can not wake a baby or stop them from having a seizure. #2-If he reacts normally after the event, alert, etc. it is not a seizure. I did tons of research with my holistic doctor and came up with this answer. I do not have the research any more (can't locate it) but it said a high amount of infants have this and many mom's do not know it because it happens in their crib. It occurs before the rem sleep pattern.
It is scary, been there done that. I feel this is what it is and LO is fine. Ask you doctor about it!