What is khushoo (sincerity) of salat?

We have all heard of the need for khushoo in salat (prayers) and other Ibadat. But, what does it mean? Here is a short explanation by Ibn al-Qayyim (may Allaah have mercy on him) who said,

“The khushoo’ of true faith is when the heart feels aware and humble before the greatness and glory of Allaah, and is filled with awe, fear and shyness, so that the heart is utterly humbled before Allaah and broken, as it were, with fear, shyness, love and the recognition of the blessings of Allaah and its own sins. So no doubt the khushoo’ of the heart is followed by the khushoo’ of the body. .” (Al-Rooh, p. 314)

Hudhayfah (may Allaah be pleased with him) used to say: “Beware of the khushoo’ of hypocrisy.” He was asked, “What is the khushoo’ of hypocrisy?” He said, “When the body shows khushoo’ but there is no khushoo’ in the heart.” Fudayl ibn ‘Ayaad said: “It was disliked for a man to show more khushoo’ than he had in his heart.” One of them saw a man showing khushoo’ in his shoulders and body, and said, “O So and so, khushoo’ is here.” – and he pointed to his chest, “Not here.” – and he pointed to his shoulders. (al-Madaarij, 1/521)