From the Super Mario Wiki, the Mario encyclopedia
An Antigravity Panel is a panel that appears in Mario Kart 8 and Mario Kart 8 Deluxe. The panel is blue with a circle pattern on it and marks the beginning of Anti-gravity sections. It must be noted that the activation of anti-gravity mode does not rely on these panels, as it is simply activated when going in parts of the track that require it, as can be noted when driving backwards or in reverse in tracks such as Mario Circuit. Therefore, their function is merely informative and unlike Dash Panels, the panels do not provide any sort of speed boost when driven over. It must be noted that characters only shout or speak the first time they activate anti-gravity mode and thus do not actually react to the panel. The panels are included in both nitro and updated retro courses, but courses that fully take place in anti-gravity, such as GCN Baby Park do not feature them at all.