What this editing action does is that TAD would create a new corner between the first and second helper. Now that a new corner was created; you will find the helpers would be relocated to the new edge that got created in the process.

Warning: If you happen to have the architect exactly on the first helper, you may wonder where the additional corner was created. Answer: It got created alright, but happened to be exactly on the previous helper. So make sure that the architect is moved away from the first helper before you invoke this command

Don't worry if you accidentally created a new corner erroneously. You can press UNDO and get the earlier shape of the object back. You can also delete the new corner that you wrongly created.

You can always locate the architect accurately using the headsup command line, before adding a corner.

Creating a chamfer
If you invoke this command, when the yellow command line is open with a value inside it; then a chamfer is created on the object. This is how it happens: The first point of the chamfer is the additional point that you added between the 1st and 2nd helper. Then the first helper gets shifted by the distance specified by the command line towards the previous point of the object. Try it out once or twice, and I'm sure you'll get the hang of it.