Understand in its ordinary sense is what is called a stative verb: one which ascribes a continuing ‘state’ rather than an action or a change of state to its subject. Some other stative verbs are like, own, consist of. The subject of a stative verb is not required to take any action or exert any effort to maintain the state; consequently, these verbs are not ordinarily cast in the continuous (progressive) forms.
✲John is not liking Mary.
✲We were owning a couple of Harleys.
✲A martini is consisting of gin and vermouth, with an olive.
✲I am understanding the Pythagorean theorem.
Some other ‘diagnostics’ of stative verbs are:
- They do not bear a ‘habitual’ sense in the simple present — ✲John likes Mary twice a week.
- They can’t be used in the imperative — ✲Own a couple of Harleys!
- They can’t be used with the ‘What happened was’ construction — ✲What happened was the martini consisted of gin and vermouth, with an olive.
- They can’t be used with ‘carefully’ or ‘deliberately’ — ✲I deliberately understand the Pythagorean theorem.
Like most common English words, however, understand has a range of senses. Ordinarily you say you understand something, and that's it; but it's also possible to understand more, or more deeply, or up to a point. In these cases it's quite in order to treat understand as something more like a process or activity, and to cast it into a continuous form:
I’m understanding Hamlet just fine up to the point where he starts yelling at his mother.
Every day she’s understanding more and more of what I say.
Employing understanding in a conditional, in an if clause, causes a similar ambiguity: maybe I understand you, maybe I don’t, it’s not clear whether I’m actually in the state of understanding you or actively trying to get into that state. In that case, the continuous form If I am understanding your question not only emphasizes my uncertainty, it also appeals to you to understand that I am making an effort.
This is neither a new use, or a colloquialism, or a colonialism: search Google Books for the 19th century on I am understanding and we are understanding and you are understanding and you will find uses in Parliamentary speeches and essays in Church of England journals as well as American religious and academic articles. No one will frown if you use it.