Q/.Is there any short comprising of 2 words/phrase that can be recited while inhaling & exhaling like mantra chanting (Bud- dho) in order to strengthen our contemplation.
A/.For myself the expression "Don't know" covers so many facets of mindfulness development, it is surprising more people are not aware of it. The Zen practice of the "Don't know mind" that 'empties' the mind, is not what is intended here; but of a meaningful enquiry so as to keep focused on just about any object of contemplation. One of the basics of Dhamma is Anattaa, hence any object under investigation will respond to the genuine exclamation of "Don't know" simply because all objects lack any substance and thus arn't objects.
Unwholesome objects will become 'stressed' or rejected; whilst wholesome objects will gladden and develop, until such time as increasing subtlety renders such objects unwholesome so as to give way to states of ever greater subtlety and wholesoeness: etc, etc.
For some practitioners the best results might come when the so called 'heart' centre is focused on, whilst synchronising the breath. The space behind the sternum joint at the first visible rib pair: but this drifts off topic.