How it works is that firms decide how much human labour is demanded.
The price of this human labour is usually not the price demanded at the labour market, but it is often higher. This is more true than ever nowadays, because hiring a human means you have to take into account:
-Their wage
-The amount of hours that they work (Also how much breaks they take)
-Benefits you have to pay them, such as pensions and stuff.
-The cost of them making a mistake.
If someone managed to make a machine that automated what a human would do, then the automation would be cheaper, because:
-Their cost is only the cost of electricity.
-They can literally work 24/7 without a break.
-No extra benefits you have to pay for the machine.
-The costs of the machine making a mistake will still be there, but the overall cost will be much smaller, because of the above points.
So because firms decide how much human labour they want, if they see this machine, a substitute labour which is much cheaper, they will prefer it over human labour.
Firms will try to maximize profit by lowering their costs, if they can. Eventually, those firms that utilize machines can lower their costs enough to offer competitive prices for their products, which will increase the demand for their product. So in order to compete, every other firm has to be either automate or leave the market.
When every firm tries to replace their human labour with automation, and they will, because they will choose the most profit maximizing condition, then you will end up with a lot of unemployment, not because those humans don't want to be employed, but because technology has allowed their job to be done cheaper.
This has happened in the past, and he forgot to mention it. We can look at Slavery in the US as a case study.
Before cotton-making/farming was automated by a cotton mill (I think that is what it was called), it required heavy labour. The US at the time needed to purchase slaves in order to maintain production of cotton, which was a profitable market.
Now, I am not saying slavery was good. Slaves lived in poor conditions, and ate little.
Eventually, once this cotton making technology arrived to the US, these farm owners realized that it would be more profitable to just maintain this cotton mill than to pay for the slaves' livelihood. Coincidentally, it was around this time that slavery was abolished in the United States, so many slaves were freed.
However, that is at least a million people that used to be working and are now out of a job. Ever since the cotton mill was introduced, they were seen as excess labour, and there was literally no job for them to do, so even though they were no longer slaves, they were probably living under the same or even worse conditions as freed people.
Obviously, this leads to another problem. If every firm does this to decrease the cost of their product, then they also decrease the demand for their product, because people who are unemployed just will not have the ability to purchase anything. So how do you balance automation so that you still have a market to sell to?