The yearly charge, even if you do not sell
This is the feature that surprises people. Under Germany's investment-tax rules, a foreign fund is subject to an annual advance charge called the Vorabpauschale. It is a small deemed return, computed from the fund's value at the start of the year multiplied by a set base rate, currently modest, and capped at the fund's actual rise in value for the year. You are taxed on that advance amount at the flat 25% plus surcharge, even on an accumulating fund that distributes nothing and even though you have not sold anything.
Equity funds soften this: a 30% partial exemption applies to the advance charge, to distributions and to the eventual sale gain, so only 70% of each is taxed. The point to take away is that an Indian fund is not fully tax-deferred for a German resident, you pay a little each year. It is much milder than the American regime that penalises foreign funds, but it is not nothing, and it needs reporting annually.
When you sell, and the India side
When you finally redeem, the gain is taxed at the same flat rate, again with the 30% partial exemption for an equity fund. Crucially, the advance charges you already paid over the years are deducted from the sale gain, so you are not taxed twice on the same growth. So the yearly charges are, in effect, prepayments against the final bill.
India taxes the redemption in its own right: equity fund long-term gains over ₹1.25 lakh at 12.5%, short-term at 20%, and debt funds at your slab rate, with the fund house deducting TDS. Here the treaty adds a wrinkle worth knowing: Indian mutual-fund units are units of a trust, not shares in a company, and for that kind of asset the treaty tends to give the sole right to tax the gain to your country of residence, Germany. So India may not have a treaty right to tax the gain at all, which can turn the fund house's TDS into something you reclaim from India, rather than a credit you take in Germany. A practising CA computes the Indian position, reclaims the TDS where the treaty removes India's right, and gives your German accountant the figures.