Regardless of how effective it is at managing expectations, my reality is:

Mgt: We need A within Y days.

Stf: A will take X (>>Y) days, A/2 can be ready in Y days.

Mgt: O.K., give us A/2 in Y days.

Y/2 days elapses.

Mgt: In addition to A/2, please add B, C and D to the deliverable.

Stf: You realize, that's going to take an extra 2X days to deliver.

Mgt: yes, but we absolutely need B. C and D, whatever it takes, as soon as possible, don't waste time releasing A/2, bundle them all together for efficiency.

Stf: Coming up.

3X/2 days elapse.

Mgt: you know B, C, and D that we asked for? Actually, what we need are D, E and F.

Stf: well, we've finished A/2, B and half of C. When do you need D, E and F.

Mgt: ASAP, put C on the shelf.

X more days elapse.

Stf: here's A/2, B and D, how do they look?

Mgt: You know, the full A would be better... but E and G are our absolute top priorities....