Leaving dependent controls enabled

In certain types of UI such as wizards and forms, it’s common to see UI like this:

Here the text box is said to be a "dependent control", because its enabled state depends upon the current selected state of the radio buttons.

In situations like the one above, I personally prefer a design that leaves the text box enabled at all times. Instead of using the state of the radio buttons to drive the state of the text box, the design goes the other way: the state of the text box drives the state of the radio buttons.

• The text box is always enabled but is empty by default.
• If the user clicks in the text box and enters any text, the second radio button becomes selected to let the user know that they have now implicitly chosen the second option.
• Clearing the text box implicitly selects the first radio button (but leaves the focus in the text box so the user can enter something new).
• If the user types something in the text box and then changes their mind to explictly select the first radio button, the (non-empty) contents of the text box are left as is. If the user goes back and changes the contents of the text box, the second radio button is again implicitly selected (as long as the text box is non-empty).

This lets the user who wants to enter something in the text box do so directly, without having to first select the second radio button. In my experience, this technique makes the UI feel faster, avoids frustration ("Why can’t I type in the box?"), and doesn’t suffer any practical downside.