If we are focused in samādhi with continuous mindfulness, then sometimes the state known as pīti will arise.
Pīti is characterised by physical sensations of coolness or of a rapturous energy thrilling throughout the body – like waves breaking on the shore – which can cause the body to sway and the hair to stand on end. These sensations are accompanied by mental perceptions of physical expansiveness. When mindful awareness is focused continuously, it can seem that the hands and feet have vanished. The feelings in other areas of the body, even the sensation of the whole body itself, can likewise entirely disappear from consciousness. The body feels completely tranquil.
During this period that the heart is peaceful, the mind temporarily lets go of its attachment to the physical body and consequently mind and body feel light and tranquil. As we sit in meditation and this tranquillity increases, it can seem as though we are floating in space, giving rise to feelings of happiness and well-being. At this point we can say that the power of our concentration has deepened to the level of upacāra samādhi.
As samādhi deepens further, the heart experiences even greater rapture and bliss together with feelings of profound inner strength and stability. All thoughts completely cease and the mind becomes utterly still and one-pointed. At this stage we cannot control or direct the meditation. The heart follows its natural course, entering a unified state with only a single object of consciousness.
This is the unification of mind in samādhi; the heart has been stilled and brought to singleness.
This reflection by Ajahn Anan is from the book, Seeking Buddho, (pdf) pp. 7-8.