@guy b. Dude, you have a way of asking short, quickly typed questions that require longish answers.
“I mean specifically to develop "internal stength/qi" or whatever you like to call it.”
Yes, let’s call it “internal strength,” as “Qi” just confuses everything and is so open to weird interpretations, we’ll end up talking in circles.
“Is there a specific targetted way of achieving it?”
Yes
“If so then this will tell you exactly what it is you are developing in physical terms.”
In brief…
The method:
(each of the things listed below have various training practices/subsets, and we work them with drills, solo work, sometimes partner work, ways of testing, etc.)
Learn to set up the frame; learn how to loosen/soften the body; learn how to align, learn how to pull and connect; learn how to wind and spiral; learn how to pair; learn how to integrate.
Inside of all of that you find the stuff and training that people in the internal arts usually reference: dantien and mingmen, breathing, intent, six harmonies, cross-body, kwa, full body power, contract and expand, open and close...
....and so on.
And it works in a cycle, too. As you progress you’re constantly returning to earlier material to refine it, further develop it, better integrate it, etc. Hence, why you never stop “basic” training like Santi, the five element fists, etc.
What you develop (in physical terms):
Releasing/getting rid of pre-existing problems (tension, mobility issues, etc.); a higher yield on alignment (because it now becomes more ingrained) that allows force/power to transmit nicely; building/developing the right/“correct” musculature, connective tissues, so that it better functions as chains, and more deeply connects the body (by binding and cutting out the slack) – allowing for greater power generation; the ability to tie all of this in with movement/interaction by pairing parts of the body (in both linear ways and circular ways) – which allows you to use much of the above in a practical way (fighting application)…
... and so on (this is all just a brief explanation, otherwise I'll be typing all night)
Intent and visualization work aids with all of the above, but also has the benefit of helping you program movement patterns (so I’d group this in with something "physical"). So intent is work based around the mind's
will to act, and the body’s ability to
actually act (mind/body connection). This is the bit where people go, “Whaaaaat….”. Feedback loops, single point training, act before you act, etc.
Or… linking thought and mechanical action/reaction to improve movement capacity...
... and so on....