Legitimate SMTP servers will try your domain's MX records one by one, in order of priority, until they reach one that accepts the message. Spammers' scripts usually don't bother, they just try the first one and move on to the next address on their list.

Of course, this is not 100% reliable, as it's not too difficult for spammers to adapt and improve their scripts. Of course, vast majority of spammers are either not sophisticated enough, or do not care enough to do so, so if you don't mind your incoming mail to be slightly delayed, it's kind of a low-hanging fruit, as it cuts off a huge amount of low-effort spammers.

Not GP, but I think what they meant is having a MX record with a higher priority pointing to an unroutable IP

    blackhole  IN A 240.0.0.1

    @ MX 10 blackhole.example.com
    @ MX 20 mail.example.com