What they do is fairly straight forward. They create an area which has the properties of both a Low Visibility Zone(or Poor Visibility Zone for Nimbus Plus ammunition) and of a Saturation Zone. This results in a penalty to hit (-3 or -6 respectively) as well as reducing the Burst of all attacks into, out of and through the Zone by 1 to a minimum of 1.
The visibility/Burst penalties cannot be bypassed by Multispectral Visors, apply to both player equally and persist until the end of that player turn.
These esoteric pieces of equipment are scattered about a variety of troops but hardly see any use at all in my experience. In fact neither myself nor my regular group have ever used them or heard of them being used in a meaningful way.
Clearly they must have an intended role though, otherwise they wouldn't be in the game right? So I got to thinking in more detail about when I might make use of these strange explosives.
We theorised that we might use it against a Fireteam where you can reduce their Burst from 2 to 1, whilst your attacks with a HMG would go from 4 Burst to 3. In theory it would affect the link more than yourself, although the -3 MOD isn't really helpful given that you have the far superior Burst.
Here are a few quick calcs (rounded to nearest whole numbers), all involving a Dakini HMG shooting at a Fusilier MULTI Sniper where both are in cover. The odds show the relative chance of you causing a wound vs them causing a wound.
We're not taking the possibility of Dodging into account and keep in mind that if your HMG is linked then the benefits should be even clearer as you'd only be losing 20% of your Burst rather than 25%.
1. Dakini HMG vs Fusilier Sniper in 5-man link. - 45% for, 32% against. (1.4:1 odds)
2. Dakini HMG vs Fusilier Sniper in 5-man link. Nimbus Zone - 44% for, 21% against. (2.1:1 odds)
3. Dakini HMG vs Fusilier Sniper in 3-man link. - 57% for, 18% against. (3.16:1 odds)
4. Dakini HMG vs Fusilier Sniper in 3-man link. Nimbus Zone - 50% for, 12% against. (4.16:1 odds)
As you can see, your own chance to wound remains fairly stable whereas your opponent's drops by about a third. That's not insignificant at all if you are in a position where you can't otherwise safely dismantle the opposing link. Is this worth spending one or more orders on placing that Nibmus Zone though? Unless you have orders to burn then I'm not sure.
Let's take a look at Nimbus Plus whilst we're at it and even though it's only available to Tohaa currently we'll use the same troops for consistency. Nimbus Plus gives a Poor (-6) Visibility Zone instead of a Low (-3) Visibility one.
5. Dakini HMG vs Fusilier Sniper in 5-man link. Nimbus Plus - 34% for, 17% against. (2:1 odds)
6. Dakini HMG vs Fusilier Sniper in 3-man link. Nimbus Plus - 38% for, 8% against. (4.75:1 odds)
Whilst it clearly makes the attack much safer, giving us good odds, it also significantly cuts our rate of success by about 25% and 33% respectively. The biggest benefit is for the latter example by far as you can really start leveraging all those cumulative hit mods!
Overall, we can see that it is clearly beneficial to put down a Nimbus Grenade in advance of trying to tackle a link team head on like this. Whether you can afford to spend an order doing so is up to you however.
For reference, here is what the numbers look like for purely a saturation zone, without any visiblity modifiers, just in case you ever felt like chucking a Nimbus Grenade whilst playing Rescue.
7. Dakini HMG vs Fusilier Sniper in 5-man link. Saturation Zone - 48% for, 23% against. (2.08:1 odds)
8. Dakini HMG vs Fusilier Sniper in 3-man link. Saturation Zone - 57% for, 13% against. (4.38:1 odds)
These are pretty much as good, if not better, than many of the scenarios above. The take away being don't bother using Nimbus in what is already a saturation zone.
Of course, there is one more benefit to Nimbus Grenades that I touched on above. Stacking modifiers.
If you can stack Camouflage/ODD, Cover, Range, Surprise Shot and then Nimbus on top then you can feasibly put your opponent in a situation where suddenly they can't shoot back at all. They'll even be suffering those penalties to Dodge too! Of course your own chance of hitting drastically goes down, but that may be a price you are willing to pay for complete safety, to break up someone's suppressive fire position or force their Sniper out of position.
9. Dakini HMG vs Fusilier HMG in Suppressive Fire. 18-24" Range. 56% for, 14% against. (4:1 odds)
10. Dakini HMG vs Fusilier HMG in Suppressive Fire. 18-24" Range. Nimbus Zone. 41% for. (Infinity:1 odds)
11. Dakini HMG vs Fusilier Dodge. Nimbus Zone. 48% for, 16% against (Dodge only). (3:1 odds)
As you can see, you go from a potentially dangerous proposition to basically forcing your opponent to voluntarily break their suppressive fire. Note that the -3 MOD for shooting against Suppressive Fire still applies if the suppressing model is hitting on effective BS 0 or less.
You can also use it to add a penalty to Dodging against Direct Template Weapons - particularly useful if you are trying to ignite an entire Fireteam with a Flamethrower or Chain Rifle for example. If your Direct Template Weapon is linked then often you'd still be able to fire it with Burst 2 as well by aiming at two different primary targets.
You could even combo a Nimbus Zone with Triangulated Fire on a unit with Sensor in order to make the -3/-6 penalty entirely one sided, although you would still suffer the penalty to Burst. There could definitely be shenanigans in Tohaa involving Keesan (who has a Sensor) and some Nimbus Plus grenades...
12. Linked Aelis Keesan Viral Combi (Tri Fire) vs Nisse Sniper. 32-48" Range, Nimbus Plus. 49% for, 15% against. (5.25:1 odds)
13. Linked Gao-Rael Sniper vs Nisse Sniper. 46% for, 21% against. (2.19:1 odds)
Or possibly not as sadly you can't make use of the Burst bonus for the link with Triangulated Fire, oh well!
So, how do you use Nimbus Zones? Would you use either of the tactics above or do you have your own secret strategies? Let me know!