This is how you do it.
You exhale about 70% of the air. The way I do it is like I take a bigger breath, then exhale it to the point where my lungs and chest naturally stops in their relaxed position. So like not forcing every drop of air out, but not trying to hold in any either. And you hold it like that. I sometimes block my nose with my hand too. It takes somewhere around 30-60 seconds until my backside relaxes enough so that poop can happen depending on how tense the situation is, and my confidence level at the time.
The downside is that it's really uncomfortable. It's not scary, it's not difficult, it's just a really really uncomfortable feeling.
I often take a watch and measure the time. As you sit there in agony, gasping for air, it's easy to misjudge the situation and decide to breath. By looking at the number of elapsed seconds, I know exactly where I am. If I use the technique for several consecutive days, I find that my tolerance level increases, and I have to wait longer, sometimes even more than a minute.
Feeling the urge for me means when I feel I need to poop, I just can't push it out. Sometimes you've got the feeling it's right there, you just can't release it. And sometimes you have the feeling that it's gone back completely. And there's the middle, in between, where you have an urge, you assess your situation and circumstances, and suddenly you completely lock up, but it's still somewhat there. For me this technique always works when I have the urge just can't release it, and sometimes works when I completely lock up but it's still somewhat there.