You have complete control over the amount of suction. If marks are being left, too much suction/duration was used on that particular bite. Some areas have thinner skin, so they require less suction (how high you pull up on the handles) and duration (how long you hold in place). On more sensitive areas or if using on children, try using the tool at half strength or less for 5-10 seconds to start. You can always increase the strength/duration or repeat the process to achieve desired results. Less is more until you are familiar with the tool.