dd is a standard unix command, but low level, like cp or cat.
If you don't already understand what the command is doing just by reading it, then you should stay far far away from it. It's a chainsaw. It can create a sculpture or cut your leg off. It doesn't know the difference. YOU have to know that, or else better not pick it up!
It doesn't flash recoveries, it overwrites files or devices (partitions). In this case the input file happens to be a recovery, and the output file happens to be a partition on the flash disk where hboot expects to find a recovery. So in this case, it happens to flash a recovery (also it's used to make a backup with a similar but reversed command), and as long as the options used are correct, and you have sufficient permissions (root), it works. But it doesn't actually know anything about hboot or recoveries or android or anything any more than a hammer knows how to build a house even though a carpenter builds a house with a hammer. It's not safe at all. If you give it the wrong command it will happily destroy anything anywhere on your phone, even the internal and external sd cards.
That's why they are saying just use the more user friendly tools instead, aka goomanager or pj75img.zip. If they don't work, report that and wait for the developer of that tool to fix it.
To understand what the commands you are reading mean, just google "dd" or "unix dd" or "linux dd" and read the "man page" for dd, and compare the command you are reading with the options documented in the man page.
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