The saksat feature of Sri Caitanya Mahaprabhu is His personal presence. Avesa refers to invested power, like that invested in Nakula Brahmacari. Avirbhava is a manifestation of the Lord that appears even though He is personally not present. For example, Sri Sacimata offered food at home to Sri Caitanya Mahaprabhu although He was far away in Jagannatha Puri, and when she opened her eyes after offering the food she saw that it had actually been eaten by Sri Caitanya Mahaprabhu. Similarly, when Srivasa Thakura performed sankirtana, everyone felt the presence of Sri Caitanya Mahaprabhu, even in His absence. This is another example of avirbhava.