With this iExpression the property is automatically increased (or decreased) by some fixed amount at each marker. In addition, you can overwrite this behavior at individual markers using the marker comments. If a marker has "-5" as comment, for example, the value is decreased by 5 at this particular marker. With a comment "5s" or "20 frames" also the fade in duration for this marker can be changed to 5 seconds or 20 frames, respectively.

• Increment at Marker

  How much the value should increment at each marker (if the marker has no comment specifying a different value).

• Frame Offset

  for -2, the modification already starts 2 frames before each marker.

• Marker Source

  Use Markers from

  whether the iExpression should use the markers of the layer where it is applied to, the composition markers, or the markers from another layer

  Other Layer (Name or Index)

  is the name or index of the layer whose marksers should be considered (if "Use Markers from" is set to "other layer").

• Fade In

  Fade In Duration

  determines the number of frames over which the change at each marker is faded in.

  Fade Type

  determines whether the change at the markers fades in in a linear or exponential fashion.

Also see the FAQ: How to create a countdown text that changes at each marker?