[Stratoscope]

When the metal is hot, solder remains melted, flows onto the metal and bonds to it. In fact, when soldering you should heat only the metal parts first and not the solder directly, and then apply the solder to the metal parts where the soldering iron touches them. You never heat solder directly with the soldering iron and expect it to stick to cold metal.

When the metal is cold, the molten solder freezes up as it touches it, making electrical contact but not really sticking. So sometime later, as a result of thermal expansion or mechanical flexing, the solder joint becomes unstuck. This is called a "cold joint".

It's also essential to use flux when soldering. Most solders used in electronic hand-soldering have a rosin flux core in the middle. The rosin melts first onto the metal you're heating, preventing the heat from forming an oxide layer so the solder can bond to the metal. (I think the flux also helps break down any existing oxide layer, not too sure on that part.)

This teacher's technique would result in unreliable cold joints all up and down the pins.

Search for "cold solder joint" for more information and photos of cold joints.

[Bob_Sheep]

It sounded like a bad description of drag soldering which involves dragging a ball of solder over the pins with the iron and relying on the solder mask and surface tension to make a good connection on each pin.

[stiff]

Yes, judging from YT videos of drag soldering, this was what he did, except unlike people in those videos, he did it holding the circuit board vertically and hence it was one very quick move. Bear with me: this was almost ten years ago, I had nothing to do with electronics since, and I am not a native speaker of English!