Sati is recollection, sampajañña is self-awareness. This awareness is the actual Buddho, the Buddha. When there is sati-sampajañña, understanding will follow. We know what is going on. When the eye sees forms: is this proper or improper? When the ear hears sound: is this appropriate or inappropriate? Is it harmful? Is it wrong, is it right? And so on like this with everything. If we understand we hear the Dhamma all the time.
Therefore it is said we must have sati. If we have satiwe will see the state of our own mind. Whatever we are thinking or feeling we must know it. This knowing is called Buddho, the Buddha, the one who knows... who knows thoroughly, who knows clearly and completely. When the mind knows completely we find the right practice.