If you look next, the majority of properties in After Effects, you'll notice there's an icon that's called the property pickwhip.

We can use this to create a linking animation between different properties in a project.

To start with our project, I'm going to click once down here in the empty portion of the Timeline to make sure that panel is active then I'll press the Spacebar to preview our animation.

And you can see as the word grows out towards the camera, the shadow gets a little bit further away from my text.

And I want to use the distance of that shadow to drive the blurriness of the background.

So, I'll go ahead and press the Spacebar to stop playback and press the home key to move back to the beginning of the Timeline.

Now, I'm going to make sure my Timeline panel is pretty large so I'll just drag up on the top part of the panel here, and if you don't already have the text layer open, go ahead and twirl open that layer and I want you to make sure the Effects are open and under Radial Shadow, you want to be able to see Projection Distance.

Now, when I scrub the Current Time Indicator, you'll notice the Projection Distance increases.

And I want the blurriness of this background to match the Projection Distance.

So, to save space, I'll collapse the Transform options here for Layer 1 and I'll select Layer 2 and press E on my keyboard to open the Effects, then I'll open up the Fast Box Blur.

Here, I want the Blur Radius to be driven by the Projection Distance.

So, I'm going to start by going to the Blur Radius here and then I'll click on the pickwhip and I'll drag, I'm holding the mouse button down and I'll hover over the words Projection Distance and then I'll let go of my mouse button and that's going to link the Blur Radius with the Projection Distance.

So, if I make my Timeline panel a little smaller so we can better see the Composition panel, now when I press the Spacebar, we'll get a preview of this current animation and it may take a second to load and that's OK, that's because it's actually blurring the background footage as the text is growing into the scene.

Now, since the Projection Distance only went to a maximum setting of 5, I may want to increase the blurriness of this background.

So, you can actually increase the amount of any value by adding math to the end of that expression that was created.

So, what I'm going to do is press the Spacebar to stop playback here and I'll scroll down to my Blur Radius.

So here, notice the number is red.

That's letting me know that there is a link between this parameter and another one.

And if I click the little arrow to the left of the stopwatch and open that up, you can see the Blur Radius over here is tied to the Radial Shadow Projection Distance.

Now, I would like the background to be three times blurrier than what's currently going on right now.

So, I'm going to click to the right of the text here just once and then I'll click one more time so nothing is selected and I'm going to add the * key which is just over top of the number eight, and that means multiply and I'll say 3 to multiply whatever number is here times 3.

So, I'll click outside of the expression area here and you'll notice now, when I scrub it down to the end of the Timeline you can see the blurriness is now set to 15.

So, if I scroll up here, you can see the Projection Distance is 5.

So, the Blur Radius is three times the Projection Distance.

All of this made possible by linking properties using the property pickwhip.