What is cooking a liquid just below boiling point and bubbles rise slowly break before reaching surface?

The term you are referring to is known as "simmering." Simmering is a cooking technique in which a liquid is heated to just below its boiling point, usually between 185°F (85°C) and 205°F (96°C), and maintained at that temperature for a period of time.

Here's what happens when a liquid is simmering:

1. Bubbles: When the liquid reaches the simmering point, small bubbles start to form at the bottom of the pot and slowly rise to the surface. These bubbles are formed by water molecules converting into water vapor.

2. Slow Break: As the bubbles rise, they move slowly and break before reaching the surface. This is unlike boiling, where the bubbles become more energetic and reach the surface more quickly, bursting on contact.

3. Gentle Movement: During simmering, the liquid maintains a gentle, steady movement with continuous but gentle bubbling. This is different from boiling, which causes the liquid to move vigorously and form large, rapidly rising bubbles.

Simmering is often used as a cooking method to gently cook delicate foods like fish, vegetables, and sauces without overcooking them. It allows for a more controlled and even distribution of heat, preventing scorching or boiling over. Simmering is also commonly used to reduce liquids and concentrate their flavors.