Why

Sweating softens and releases flavors using moisture and low temperature.

How

Covering the pan and heating over low temperature with frequent stirring.

The fat holds “non-volatile flavors” as they’re released. The lid traps the steam, which condenses and drips (onto the onions).

Tender, translucent pieces. With no browning.

In Italy this technique is known as soffritto. Which means sub-frying or under-frying.