Changing Gears means that you adjust your level of aggression and playing style based on the situation.
The best players in the world are often the players who can change gears at the right times.
THE BASICS OF CHANGING GEARS
Your style should be the opposite of your table’s style: When your table is aggressive, play tighter. When your table is tight, change gears to a more aggressive style.
Play counter to your image: If you are perceived as tight, loosen up for a little while. When you look like you are a looser player, tighten up.
On bubbles: When you are on big money bubbles, or the final table bubble, it is important to try to change gears to slightly more aggressive style to exploit the players trying to play tight and move up the pay ladder.