When the baby is able to use their magic whilst in the womb to protect their mother
Baby in womb
Power Tapping occurs in magical pregnancies where psychic bonds between the child and mother are created. In this situations the baby is able to activate its powers to protect its mother when it senses she's in pain or danger. In some instances the mother can consciously tap into the powers of her unborn child.
Whilst Charlotte was pregnant with Melinda, she had premonitions from the womb. With this ability, Melinda gave her mother a premonition which told her that her lover, Lawrence, was never coming back.
Whilst Patty was pregnant with Phoebe, she had also premonitions from the womb. When Patty had a premonition of future Prue, Piper and Phoebe, she thought they were warlocks and warned her mother. She later tapped into Phoebe's powers to use a Power of Three spell to send the sisters back to the future. At Piper's wedding, she revealed that the day of Phoebe's birth, she had a premonition of her hugging her daughters, on that very day.
Whilst pregnant with the Source's heir, Phoebe shared a magical bond with the fetus growing inside of her. Through their bond, the unborn child was able to cause Phoebe's head to emit flames while she had a fever at the hospital. While performing an ultrasound on Phoebe, the baby was able to channel its power through its mother and electrocuted the doctor in an attempt to protect itself from the cold ultrasound gel.
The child was also able to cause Phoebe to feel animosity towards Paige and provoked her to commit acts of violence on her. An instance of this would be while in the attic of Halliwell Manor, the unborn child caused Phoebe to send her sister Paige Matthews flying out the attic window. Another instance would be when the child caused Phoebe to punch Paige in the face while at Cole Turner's Apartment after Paige cast the Vanishing Spell to erase remnants and traces of Cole's vanquish. The baby did not like hearing this and sought its vengeance for what was said about its father, causing Phoebe to also call Paige a "murdering witch".
The unborn fetus also allowed Phoebe to tap into its power of Fire Throwing, letting Phoebe send a stream of flames at whatever object or being she chose to. After Paige created the vanquish potion to destroy Cole, she told Phoebe and the baby caused Phoebe to shoot a stream of fire at Paige knocking the potion onto the floor.
After Piper Halliwell became pregnant with her first son Wyatt, she displayed concern for the safety of their child and soon discovered that the unborn baby could heal Piper's wounds from inside the womb.
When Piper was desperate to talk to someone who had experience in raising and carrying a magical baby, she was frustrated for being unable to summon Grams. The baby could feel the frustration felt by his mother and during a heated argument with Leo, Grams was magically summoned in the room and she was made corporeal. Leo theorized that the baby conjured her to help Piper.
When Leo and Piper started to have communication problems, they began to argue constantly. Piper's frustration over Leo not being there more for her pregnancy and Leo feeling as if Piper doesn't understand the importance of his other charges and Whitelighter duties fueled these arguments. When Piper and Leo began arguing again, the baby switched their powers as well as giving Leo Piper's pregnancy symptoms so the two could understand how each other feels. He also repeatedly interfered with Piper's power of Molecular Combustion, turning it into fireworks and flowers when she tried to blow up the Siren. After the Siren was vanquished by Leo with Piper's blasting power, he switched them back and stopped interfering with Piper's power.
An encounter with a Darklighter led to the discovery that the baby could shield itself from demonic attacks. When the Darklighter attacked Piper and sent an arrow toward her, the baby sensed the threat to his mother and created the shield, deflecting the arrow away from Piper, leaving her unharmed. This led Piper and her family to believe she was invincible.