Reverse the polarity of the neutron flow

"Reverse the polarity of the neutron flow" was a phrase which the Doctor often used.

The Third Doctor once suggested to the Brigadier that the phrase was meaningless technobabble. (AUDIO: The Paradise of Death) With similar uncertainty, the Twelfth Doctor once "bet" that the phrase meant something and that it "sound[ed] great". (TV: The Girl Who Died)

The Brigadier told Kate Stewart that the Doctor used the phrase when he did not want to explain something. Because of this, she realised that the Eleven's description of the function of a circuit for a time field inhibitor meant nothing. (AUDIO: Time of the Vulpreen)

The Third Doctor once reversed the polarity of the neutron flow to destroy a Sea Devil base. understood the implications of his actions when the Doctor told him. (TV: The Sea Devils) The Third Doctor performed the action again whilst with his other incarnations, once in the Death Zone to free the TARDIS of a force field (TV: The Five Doctors) and once while they all helped the Fourth Doctor when he was trapped in his TARDIS. (WC: Doctors Assemble!)

The Brigadier joked that Kaleidoscope "reversed the tonality of the music flow" when he banished Keeth Hazel's demons with a reversed guitar riff. (AUDIO: Kaleidoscope)

The Fifth Doctor reversed the polarity as part of a process to stabilise his post-regeneration. (TV: Castrovalva) He also reversed the polarity to aid in his and his companion's escape from Mawdryn's ship. (TV: Mawdryn Undead)

The Ninth Doctor claimed that Adam Mitchell reversed the polarity while helping him defeat the Bygone Horde. (AUDIO: The Other Side)

The Ganger clone of the Eleventh Doctor stated a mangled version of the phrase when reliving the memories of his predecessors. He went on to say that he had "reversed the jelly baby of the neutron flow", a trademark item of the Fourth Doctor. (TV: The Almost People)

The Twelfth Doctor once reversed the polarity of the neutron flow of a Mire helmet so Ashildr could wear it to create hallucinations of an unbeatable threat, forcing the Mire to retreat. (TV: The Girl Who Died)

The Thirteenth Doctor reversed the polarity of the neutron flow between two of Professor Jericho's psychic helmets placed on a pair of Weeping Angels from the Division extraction squad, saying it should give them a quantum headache. (TV: Village of the Angels)

While not neutrons, polarity was reversed by the Third, (TV: The Dæmons, The Time Monster, Frontier in Space, Planet of the Daleks, Invasion of the Dinosaurs) Fourth, (TV: City of Death) War, (TV: The Day of the Doctor) Tenth, (TV: The Lazarus Experiment, The Day of the Doctor) Eleventh, (TV: The Day of the Doctor, The Time of the Doctor) and Thirteenth Doctors, (TV: It Takes You Away) as well as Nyssa (TV: Arc of Infinity) and Clara Oswald. (TV: Hell Bent) Yasmin Khan was also familiar with the concept. (TV: It Takes You Away)

Jean-Paul Savarin suggested that Louise Rix reverse the polarity of the Hoplite Frequency. (AUDIO: The Last Line of Defence)

Behind the scenes
Although remembered as the Third Doctor's catchphrase, "Reverse the polarity of the neutron flow" was only uttered three times by him in performed media in The Sea Devils, The Five Doctors, and Doctors Assemble. Notably, only one of these instances was during his own era. He much more commonly stated simply, "Reverse the polarity".

The action does have some basis in real world science.