Dynamic Block Creation (Symmetrical Stretch)
Add two stretch actions for parameter, and select the two grips of the two parameters as the relevant point of each action.
1. Add Stretch Actions:
In block editor, pick the linear parameter, then click entry frame which is in the right side of "distance type" on Properties panel, there comes one drop-down menu.
2. Modify Base Location:
In the misc, modify the base location from "Start Point" to "Mid Point", like the shown picture below. Save and quit the block editor.
3. Test Dynamic Block:
For the convenience of watching the effect, we draw a vertical center line at the middle of the keyway, and then stretch the grip to the left side of the keyway or the right side. Like the shown picture below. As the grips moving, two-way symmetrical stretch realized.