One of my big weaknesses with the game...
I don't know exactly where you stand with whip experience. You are a good player, so I assume you use it some, but just don't monitor the Overflow (OF) aspect of it much, so I'll offer an explanation here as basic as possible.
On normal speed, a citizen is worth 30H. When whipping, the OF counts the 30H from each citizen you whip, plus base hammers of the city at that point, any hammer modifiers (forges), civics (OR) or trait bonus on certain building if applicable. This added to the build item at the point of the whip plus any hammers already slow built into that item. (edit: I should correct here that hammer multipliers and bonuses are applied to OF the following turn)
Early game, units like Axes can be 2 pop whipped after 1t of build, if you only put no more than 4 hammers into them (pre-forge). So you need to adjust the worked tiles in a city to make sure only a minimal amount of hammers go into it. Depending on the city size and workable tiles affects how much overflow as this affects the amount of base hammers going into the build after you whip. You can often get in the range of 25 to 29 hammers which will complete another axe, spear or archer the next turn - again depending on the makeup of that city, if it's is low hammers it may take another turn to finish the new item.
This is a really fast way to get up an early army. If you have enough food and/or granaries, you can pump out a very nice army in just a few turns. Especially nice if you lack forests or already used them.
The key to this process is identifying and setting up the optimal whip points. BUG city screen enhancements help with this a lot. Sometimes you may need to adjust tiles on Turn X to make sure you reach or don't surpass the optimal whip point on Turn Y.
For instance, in Eridu, the Gran has 14 hammer into a 60H building. Eridu is producing 14 base hammers. If you go past 30H into a 60H buiding, then it is a 1 pop whip as you need less than or equal to the 30H you would get from 1 citizen. So to calculate at this point you have:
60H (Gran) - 14H (accrued) = 46H remaining to complete build
Note: Although Eridu has 14 base hammers, you will likely have less than that # after a whip depending on the city. I always whip and then adjust citizens to max hammers on that turn.
So,
60H (2whipped citizens)
- 46H (remaining in build)
= 14 hammers whip OF
+8H max base hammers in Eridu after whip
= 22H total OF next turn
However, if we wait until next turn to whip, we add 14 base hammer to the Gran build for a total of 28 hammers into the Gran. 28H does not exceed the 30H limit to 2pop the gran.
So,
60H (gran) - 28H (accrued) = 32H left to complete build
60H (2 whipped citizens)
-32H (left to complete)
=28H whip OF
+8H max base hammers in Eridu after whip
= 36H total OF
You've now just whipped a Granary sooner and completed a Vulture the next turn
We can do the same in Uruk with a Vulture whip. However, we can't do it this turn because of the small OF you get with some build completions. So always look at your production bar after a build before determining if you can setup a unit whip. ( I assume you use BUG...the whip button and production bar in city screen help immensely with these quick calculations)
So, next turn, if we switch the mines to food only tiles, we can 2 pop whip a Vulture for in excess of likely 30H OF next turn. You just completed 2 units in 2 turns.
I know it probably sounds complicated from the mess I wrote above, but it really is quite simple once you get the knack for it. V is good with this too and probably can explain it much better..
Note that when you whip an item the production bar adds the hammers to the existing hammers that turns such that it looks something like this in the case of a granary:
88/60
88 being the total hammers produced by the whipped citizens + accrued hammers
60 being the hammer cost of the time
88 - 60 = 28 whip OF
However, 88 does not included the base hammers of the city, so your total OF could be a bit more.
There are penalties and caps to OF. Also, OF hammers are subject to the hammer multipliers, civic bonuses, and trait production bonuses.
So, for instance, if I'm playing a leader with the IMP trait I will often 2 whip a worker into a settler as the IMP trait will max the OF into the settler (vice versa if EXP) . If creative, I will whip....say...a settler and use the OF into a Library
Additional note: Warriors, Archers and Chariots can obviously never be 2popped due to their low hammer cost, but I assume you know this already.
And this folks, is why we often harp on getting up granaries early and why they are so important