Phantom tingling is a sensory illusion, similar to an optical illusion. Similar phenomena include the sensation of surprise, or flenching if something appears as if it is going to hit you, your skin crawling, and ASMR; expectation causes the sensation. It however can be very useful for meditation due to the nature of the illusion.
The fact that it is an illusion can be demonstrated in two ways, as well as give a replication for people who haven't experienced this. Take a pencil or other pointed object and point it at the center of your forehead above the ridge of your brow. Move the tip close to your forehead without touching, but imagine what it would feel like if it were to touch. With luck, you'll feel the sensation straight away. If you have difficulty, try moving the pencil, doing this exercise in a mirror, or have someone else point the pencil at your forehead.
Another way to replicate the sensation is to take a coin and place it on the middle of your forehead, with your head tipped back so it doesn't fall off, but not too far back. Wait a minute and then move your head forwards, letting the coin fall. You should feel a phantom coin sensation remaining.
While possibly related to ASMR, I find that this experience is distinct; ASMR feels pleasurable while this feels neutral. While this sensation can be first experienced by props, as others have noted you can induce it just by remembering what it feels like and expecting to feel it. The sensation can also be strengthened and manipulated by concentrating on it, which is why it is useful for meditation: it gives you something to focus on instead of letting your thoughts wander, and serves as a barometer for your strength of focus. By closing your eyes and meditating by trying to focus on the sensation and nothing else, you can get into deep meditative states, including ones that give roiling colorful geometric hallucinations similar to when you are just about to fall asleep or press on your eyeballs (Don't do this, apparently it's very bad for them by increasing fluid pressure in them.) This state is exactly the same as ones reached by other methods such as breathing, visualization, and mindfulness. The deeper the meditative state, the stronger the phantom tingling.
This sensation is also not just limited to your forehead, for me it is found just as strongly in the fingertips, but to a far lesser extent can be induced in other parts and throughout the body.
To induce it in your fingertips, lay down on your back and prop up your upper arms with your elbows to your sides, and make a cage with your fingers, with your fingertips of each hand touching. Position your arms to balance them to use the least amount of exertion and present the least amount of pressure against your fingertips as possible. Now, concentrate on the sensation of your fingertips touching, recalling the sensation you have felt in your forehead. You should begin to feel the same sort of tingling. You can do this in conjunction with focusing on your forehead, but you don't have to.
Moving your hands apart won't stop the sensation, and with practice you can induce it without touching fingertips, or even while walking etc. One trick is to just slightly begin to move the muscles of your arms as if moving your fingertips apart, but keep them touching, and imagine your hands moving apart. If you are successful, it will feel as if your hands are moving apart when they are still touching, because you are tricking your mind into perceiving that they are. This seems to give an instant deepening of one's meditative state.
I would recommend not being suckered into the fantasy that this is some type of "energy," magick, or the inner resonance of your pine eel chakra colliding with the fifth moon of the solstice. Charlatans love to use this phenomena to sucker people into their hogwash, and they're in this thread in full force. The true magic is in the mind-hacking you are doing, which is entirely internal to your mind, and the potential psychological benefits of meditation which it can facilitate.