Well, this wasn't a discovery, as much as a simple application of mathematics. Given a set of 63 or 64 things (hidden resources), by creating sets and using combinations we can extend their use quite a bit.

If you have four resources, alpha, beta, gamma, delta, you can get four uses out of them, normally. However, there are lots of complex conditionals, as we have discussed elsewhere. Let's see an example of them in use.

Province X has HR alpha
Province Y has HR beta
Province Z has HR alpha, gamma, and delta

Unit 1 has the following requirement string:
requires factions blah and hidden_resource alpha and not hidden_resource beta and not hidden_resource gamma and not hidden_resource delta

Unit 1 can then be built in Province X, but not Y or Z.

Using these combinations, through a simple application of mathematics we see that each set of resources provides us with a multiplier of 2's worth of HRs.
2 = 4
3 = 8
4 = 16
5 = 32
...and so on.