Question: Handbook of MBS (SN 116)

[Will Durant]

page 311 says

The chief benefit of a tight window is the superior roll down the yield curve it provides as it ages.

I don't understand why this is the case. Any insights?

[Will Durant]

Follow-up question.

Page 329 says

The coupon-paying bonds, Classes A, B and C, receive payments of interest at their stated coupon rates on their original principal balances.

This doesn't make sense to me. I thought these classes received interest on their outstanding principal balances.

[Will Durant]

Another follow-up question (another crappy Fabozzi book)...

Page 332 in discussing the benefits of including a Z bond in a structure states

Furthermore, since the timing of cash flows from the Z bond depends on when the earlier tranches are retired, the Z bond itself also is more stable.

More stable than what? The alternative being compared against is the structure with no Z at all.

ETA - I think I have answered my own question...

I believe what they meant to write (not what they actually wrote) is that in addition to supporting earlier tranches, the last tranche in the structure is itseld more stable as a Z than it would be as a non-Z.