Usufruit is the right to use and receive the income from a property or from an investment for the rest of one's life.

If the deceased had bought the usufruit of a house, and someone else had bought the bare ownership rights with him, of if the deceased had sold the bare ownership rights to him/her, this could be a tax efficient scheme to pass on ownership. 

That is why the taxman may presume that there was a donation in disguise. He may charge inheritance tax over the assets as if they were left by the deceased in his estate.

However, the "beneficiary" may prove that it was not a disguised gift, e.g. because gift tax had been paid on the donation.