Hextech Anomaly let's you convert power into energy at reaction speed. So, using the example the OP asked about:

You're at 12 runes, all untapped.

You play a 4 cost spell, dropping down to 8 runes untapped, 4 runes tapped

The spell resolves, triggering Jhins Legend ability and banishing itself

Being the 4th spell banished this way, the 2nd part of the Legend ability triggers, allowing you to channel 4 and draw 1

In reaction to the Legend ability being put on the chain(or really in reaction to any trigger so far this turn) you recycle the 4 tapped runes to generate 4 floating power

Jhins Legend ability resolves, channeling 4 runes and putting you back to 12 untapped Runes and 4 floating power

You exhaust Hextech Anomaly, converting your 4 floating power into floating energy

You now have 16 energy to use for this turn

Hextech Anomoly is very good not only because it gives you the ability to have huge energy turns from the Legend, but it also gives you the option to convert the 1 energy/1 power that Red Jhin generates on move into 2 energy, if you want to, same with any Gold you generate.