The main idea and focus of special needs planning is to keep the child's eligibility for government benefits and services such as Medicaid even if you are transferring property for the benefit of the child. For example, if your child qualified for government benefits during your life time, but at your death you gave your estate to your child with your will. Your child may now no longer be able to qualify for government benefits because of the new property your child (or your child's guardian) now owns.
A special needs trust, sometimes called a supplemental needs trust ("SNT") is designed to give the trustee power, and place special restrictions on the trustee, over the distributions from the trust to your child. The distributions should be in a manner that the child won't lose any government benefits.
Typically, an SNT is formed by a will as a testamentary trust or as a "sub-trust" of a trust. So, typically, there is no need to form the trust until time of death; we simply put the "framework" for the trust in place now but don't actually create the trust in the present.
Just to be clear on the earlier sentence, we still need to do special needs planning now; but, the special needs trust will only begin its existence at time of your death. The planning is putting the proper structure into place, ready to spring into existence at the time of your death.