The phrase "open flap" means to lift or move a part of something, like a cover or a door, so that what is behind it can be seen or accessed. Full definition
When it is time to breastfeed, you simply open the flap on the cup so that you baby can nurse while the rest of the bra stays in place. (easybabylife.com)
The quote marks are needed, because opening this flap reveals fillers for coolant and oil and a couple of heat - extraction fans, but no engine. (roadandtrack.com)
Called heterochronic parabiosis the method involves surgically linking the circulatory systems of two mice of different ages by opening a flap of skin on each mouse's side and stitching the two together so that the same blood pumps through both creatures. (sciencemag.org)