Not on the 330(am on 320/1) but presume its the same...
Basically prior to a flight you enter a cost index(CI), a number assigned by the company based on fuel cost at dep/dest time/speed requirement etc. CI 0 equals really fuel efficient, CI99 equals goes like rocket and BP smile.
With a typical cost index in the 30's/40's you cruise at .77/.78
and descend at 280-300 knots. If youre late or need to make closing time at the local you can alter any/all of climb,cruise,descent speeds. Altering the cost index does them all. Typically in the 320/1 you can cruise at selected speeds of up to .81
and descend if you want at 340 kts (350 if youre brave/silly
).
Some companies/skippers dont like to mess with the FMGS speeds, some never use them and fly selected speeds everywhere.
Hope that helps...
Oh one other thing, and you may well know it but I'll mention it anyway. The A320/1 has an incredibly clean wing compared to the 737. This means it glides really well. If we are doing 340 knots in level flight below FL100 it takes around 10 miles to slow up to flap extending speeds (230kts) - in level flght. If were descending we cant slow down in any real sense, below 250kts there is gear etc but its messy. Slow down or go down - not both.
The 330 I hear is even better, it glides like a dream (canada to funchal I believe
). No seriously I believe it really is clean so might explain why it gets lower quicker, so it can then slow up...