Snap Toolbox Icon:

A toggle function that turns Running Snap Mode off or on, and specifies which snap locations are anticipated as the cursor moves.

NOTE: Some settings in the Preferences will also affect the Running Snap preview calculations. If you plan to use Running Snaps a lot, you may find it more efficient to turn off the Preference options for snapping to entities in a block or symbol or lines in a hatch pattern. Running snaps may also cause difficulties in 3D mode, especially when 2D Selection Mode is active.