There are many categories of trusts in the tax rules, but fundamentally two large categories, testamentary trusts and and inter vivos trusts, so trusts created by will or estate, and all other types of trusts.
Under the existing regime, essentially any trust created by a will qualified as a testamentary trust, so the tax planning that was being observed was taxpayers creating many testamentary trusts for one estate under the same will. In effect, who pays the tax under those circumstances are the trustees of the estate on behalf of, really, the beneficiaries of the estate. Under the pre-existing regime, all those testamentary trusts qualified for the graduated rate structure representing quite a large tax saving for a more sophisticated estate plan.
The other aspect of the tax planning was to leave those trusts in place for many years, much longer than required for the purposes of winding up the estate or resolving the affairs. Under the new rules, the only graduated rate estate, so one estate per deceased individual, for up to 36 months will qualify for graduated rates. Under these rules, essentially, still the trustees of that graduated rate estate are responsible for the payment of the tax, but the major change is the time limit and the imposition of one estate per deceased individual.