Tongue thrust also know as a reverse swallow is a muscle pattern which is visible by the tongue noticeably pushing forward during speaking and swallowing. This thrusting pattern is usually a compensation for other underlying issues which can or may include:
Tongue tie, oral habits (thumb sucking, pacifier use, etc), prolonged bottle or sippy cups, enlarged tonsils / adenoids, nasal allergies, and difficulty swallowing.
Over time the light continuous force of the tongue resting forward and down, or low and out, will cause the teeth to move into an unfavorable position. It is important to note if the tongue thrust is not addressed before braces are removed, teeth will shift back over time.
Leaving a tongue thrust untreated can lead to teeth shifting after braces are removed. Your muscles working properly are your faces' braces!