Ambiguity is part of Beckett's existentialism, it provides us with the possibility of a state of flux or plasticity, a point in the text where qualitative change is possible.
The subtle changes and oppositions can provide solution or advocate hope and resurrection. The apparent contradiction lies in the argument presented only by Vladimir that it is not enough to live or die. The synthesis is provided in a unifying view by Estragon who reinforces the need to give a voice to the dead. The dead are also the apathetic who can rise up out of stasis. The lifeless eternity of 'Sand' at the beginning can therefore become 'Ash' along with mythical possibilities of the Phoenix which can arise from its stasis of death to a new life.
The part immediately preceding the extract I have described deals with the ambiguous barrier between life and death:
VLADIMIR: You're a hard man to get on with, Gogo.
ESTRAGON: It'd be better if we parted.
VLADIMIR: You always say that, and you always come crawling back.
ESTRAGON: The best thing would be to kill me, like the other.
VLADIMIR: What other? [Pause.] What other?
ESTRAGON: Like billions of others.
VLADIMIR: [Sententious.] To every man his little cross. [He sighs.] Till he dies. [Afterthought.] And is forgotten.
ESTRAGON: In the meantime let us try and converse calmly, since we are incapable of keeping silent.
VLADIMIR: You're right, we're inexhaustible.
ESTRAGON: It's so we won't think.
VLADIMIR: We have that excuse.
ESTRAGON: It's so we won't hear.
VLADIMIR: We have our reasons.
The effect of ambiguity on meaning can be seen in the number of possibilities in the words, language and structure of text which can interrelate to give different meaning. The frame provides reason as well as challenging the audience to apply its own reasoning. Tension in dialogue allows for dialectical struggle between opposites and synthesis of meaning. Words as signs and signifiers can also change meanings. Dead leaves and the eternity of sand can compliment just as live leaves and sand can argue and synthesise or, live leaves and ash can compliment or dead leaves and ash can argue and synthesise. The effect can be one of stasis or motion and resolution. Each possibility can have its effect on the nature of life and death and its relationship to one another. The overall effect seems to be trying to focus on the reason of being and whether the independence of mankind can control its own destiny.