Allow keyframe spanning is enabled on the timeline by clicking on the icon that is on the right side of the Generate keyframes while tweaking. With this enabled, when you create a drag selection and you modify a value in a plugin then everything in the selection gets modified the same. It uses the previous keyframe and if there is no previous keyframe, then the default keyframe in your $HOME/.bcast5 definitions is used.
This is particularly useful in plugins that have the Drag box enabled. You can select the area on the timeline where all of the keyframes exist with the drag enabled, turn on Allow keyframe spanning, and then modify the menu for that plugin to turn off the drag.
The CINELERRA-GG Community, 2021