I believe he is referring to "Manufactured Spend". So with this card, you need to spend 10K. If you don't have that much stuff to buy, you have a problem. MS is when you buy stuff that you can convert to cash. Sort of like Money Laundering but legal. Now in this case, technically you can just spend 10K and refund it all and it will still count. MS is mostly for the cash back or points. You can see how repeating this cycle over and over again with many credit card offers can earn you tons of points at little cost.

For example, upthread, there was a reference of returning flights. You might buy a bunch of flights, then cancel them. Your money will be stuck with the airline in the form of credits but if you fly alot, maybe it's not a big deal. Similar to buying lots of gift cards.
The really good MS people can actually convert them back to cash. For example, buying gift cards that can be converted to cash. This is becoming more and more rare and credit card companies are cracking down. I'm no expert.
There's a whole forum on MS.
I think his warning is that you do it at your own risk. Never spend more than you can afford (for example if after buying the gift card, something changes and you get stuck with them)