Wow that guy's pretty good. Except that garbage about holding the fort. Why would someone want to hold a fort, and how would you fit it into your grip in the first place? You hold down a fort, by standing inside of it. Oh well, I've got a new youtube subscription... Thanks!
In military parlance from approximately the middle ages in UK history, holding a position (be it a fort, dugout, road, etc) has been taken to mean keep this position from enemy hands. Only in recent history (approx 1900-1916?) did the Americanism of "holding it down" come in to use (and it looks like no-one knows why.
Holding something down implies that, if the downward pressure of you holding it down is removed, the object will take off. With that in mind, holding down the fort is hardly a requirement as it is highly unlikely to disappear skywards if you do remove yourself from it.