What the professors-and-teachers article does
The article rewards a genuinely temporary academic visit. Where it applies, an individual who was a resident of one country immediately before visiting the other, and who visits to teach or carry out research at a recognised university, college or educational institution, is exempt on that teaching or research remuneration for a period not exceeding two years from the date of first arrival.
It works in both directions. An Indian professor who takes a two-year post abroad can be exempt in the host country on the teaching pay. A foreign or OCI professor invited to teach in India can be exempt in India on the Indian teaching pay for up to two years, claimed on the Indian return. The exemption is specific to teaching and qualifying research income, not to other income the person may have, and research qualifies only if it is undertaken in the public interest rather than primarily for the private benefit of a particular person.
The article number and the exemption are not the same in every treaty
This is where people go wrong by copying an answer from a different country. The professors article is not standardised. In the India-US treaty it is Article 22, in the India-UK treaty it is also Article 22, and in the India-Australia treaty it is Article 20, each giving a two-year exemption. The India-Canada treaty has no professors or teachers article at all, so a visiting academic in Canada falls back on the ordinary employment or independent-services rules and gets no two-year teaching exemption.
So the first thing to pin down is the exact treaty and its article, because assuming Article 22 everywhere, or assuming the exemption exists everywhere, produces a wrong position. The safe approach is to read the specific treaty for your country before relying on any exemption, which is part of what a CA does when setting up the claim.
The two-year clock and the traps that void it
The two-year limit is stricter than it looks. The exemption is for a visit not exceeding two years, and the widely applied interpretation is that if you stay beyond two years, the exemption is lost for the entire period, not just the extra time, so the first two years' pay can become taxable in hindsight. That makes an overstay expensive, and it is the single most important thing to plan around.
There are two more limits worth knowing. Research only qualifies where it is in the public interest, so research done primarily for a private party is outside the article. And the exemption is a treaty position that has to be claimed properly, with a tax residency certificate and the treaty relied on, not simply assumed. For a foreign or OCI professor teaching in India, all of this lands on the Indian return, which is squarely India-side.
A worked example: Dr. Menon teaches for a year in India
Dr. Menon, an OCI who lives and teaches in the United States, is invited to teach for a year at an Indian university on teaching pay of about 24 lakh rupees. He was a US resident immediately before arriving and the post is at a recognised institution, so under Article 22 of the India-US treaty that 24 lakh can be exempt in India, because the exemption runs for up to two years.
His CA fixes his residential status for the year, confirms the visit qualifies, and claims the Article 22 exemption on his Indian return, supported by his US tax residency certificate. Because his stay is well within two years and the work is teaching, the exemption holds and India does not tax the teaching pay. Had he been going to Canada instead, there would be no such article and no exemption, and had he planned to stay a third year, the exemption could have been lost for the whole period. The India-side claim, the residency call and the two-year planning are what we handle.