The two do relate. A bypass trust is a trust used to "bypass" the surviving spouse's estate. It is done to save estate taxes, so that the estate tax exemption of the first spouse to die can be utilized, rather than passing all assets to the surviving spouse tax free, and then paying a big tax when the surviving spouse dies. A disclaimer will is one that leaves most if not all assets to the surviving spouse, but contemplates that the spouse, for tax reasons, might want to disclaim assets the survivor inherits. If the survivor disclaims a portion of the assets, the will can provide that a bypass trust is beneficiary of those assets. The suriviving spouse may still receive income from the trust.
Because the surviving spouse is not beneficiary of the assets, they are not part of the survivor's estate when the survivor dies.
Disclaimers and bypass trusts are used because it often is impossible to predict what needs to be done to minimize taxes until one spouse dies.
No more lawyer stuff, no more political stuff, so no more CYA