Pull Form 2210, and walk (very carefully...) thru page 3 & 4.  It should clarify things much better than words can.  It is actually very logical.

Another approach is to use a dummy client, make it have a liability, then play with the numbers coming from withholding and from estimates.  Change the dates on the estimates (make some late, or even zero) and review the F 2210 page 3 & 4 and worksheets until you understand the impact.