One of the aspects listed was the ability to challenge a will due to the will maker not making the document of their own free desire.
While there will always be a myriad of influences on someone when they decide to make their will, a will is only invalid if any such influence is determined to be 'undue'.
Undue influence is said to exist when there is something so strong that the will maker is under the belief that while the document is not what they want, they feel compelled to sign it anyway.