When onion skin cells are placed in a hypertonic solution, the cells lose water and shrink. This happens because a hypertonic solution has a higher concentration of solutes (such as salts or sugars) than the concentration of solutes inside the onion cells. As a result, water moves out of the cells by osmosis, causing the cells to shrink and the cell membrane to pull away from the cell wall.
In other words, the hypertonic solution has a lower concentration of water molecules than inside the cells, so water molecules move out of the cells in an attempt to equalize the concentration of water on both sides of the cell membrane. This movement of water out of the cells causes them to shrink and become less turgid, which can affect their ability to function properly.