Why should you swing low?
Let's say your detector can go 8'' deep. If you are 4 inches off the ground, how many inches in the ground can you detect? This could be an SAT question haha. You can only go into the ground 4 inches. So, the lower you are the ground the further into the ground you will penetrate.
When I detect, I am always 1 inch or less above the ground. On the wet packed sand I am nearly touching the ground and on deep targets I will touch the ground when I go over the target to confirm its identity.
Do a test to confirm this. The next time you are out detecting and find a target, slowly swing and lift your coil above the soil. See how many inches off the soil/sand you can get before you lose the target. Do this on a deep target too. All of a sudden you realize how even holding your coil 2-3 inches off of the ground can cause you to miss targets. Its very eye opening.