Flaps do 2 things: smaller flap deployments increase lift without increasing drag much; larger increase drag without increasing lift much (beyond that of using flaps at all).

You want to be going as slow as possible when you land (aircraft make poor racecars), and most aircraft call for some level of flaps on landing. Increasing lift lowers the stall speed, which is good, and increasing drag allows you to lose altitude without gaining speed. Without flaps they would have to land too fast.

Advertisement: https://www.av8n.com/

Flaps do lower the stall speed, but they also make the glide ratio worse, so you descend more steeply.

But that is a side effect of flaps. So what you are saying is that they had planned their pattern assuming they would be able to extend flap on final but couldn't and ended up too high?

I assumed they came in a bit too high as an insurance (you can burn height but can invent it), but just ended up with a lot more than they expected just because if their lack if familiarisation with the situation in that plane, so much that the dive break alone weren't sufficient to burn it.

Yes, typically you'd want to come in a bit high (or fast; you can always trade speed for altitude to an extent).

Then, you put in flaps to have a less efficient wing profile and descent more steeply (without gaining excessive speed).

However, here they did not have enough power to do that, so the Captain put it into a slip: flying uncoordinated, with (say) left rudder, but right aileron, flying somewhat diagonally (or rather, flying straight ahead still towards the runway, but presenting not only the nose, but a bit of the aircraft body to the wind).

That gets you down quite well.

Those are really the techniques when you're too high on the approach: 1. Flaps 2. Slip 3. S-Turns

They are, but as I understand it the RAT doesn't generate enough power to operate the flaps in general once you get down below a certain speed.