Explanation of "Speed Instability on the Approach"
Most aircraft have a minimum total drag speed. Usually min drag speed for light aircraft is 10 to 20 kts above the stall and is the speed which gives you shallowest glide angle. Good to know it if ever you want to glide furthest in still air. Glide at speeds above or below min drag speed and the angle of glide increases.
Approach speeds are usually above min drag. Thus if you are below glide slope and inrease alpha you will go back towards intended glide slope, losing a little speed in the process. Drag is reduced down towards min drag speed and all feels natural as you regain glide slope..
High performance military aircraft often have min drag speeds well above an optimum speed for landing approach. With such aircraft below glide slope when you increase alpha the speed washes off fairly fast and the increase in drag can be large. Result is you don't get back to the desired glide slope and markedly increase the angle of descent. Pilot work load is high and many have auto throttles to keep you on speed.
Angle of glide is the cosine (I think) of drag over lift.
An F111 with wings fully swept to 72 degrees (26 degrees forward) has a min drag speed over 300 kts and a steeply rising drag below 300. Full dry power may not be enough to dig you out of the hole unless you can trade off altitude
Is that enough of an explanation?