An important concept of Mahayana Buddhism which is not there in Theravada is that of “non-abiding nirvana” or apraḍiṣṭhita nirvāṇa in Sanskrit. The word ‘apraḍiṣṭhita’ actually means ‘non-stationary’ or ‘not fixed in one place’ or something like that, so the translation as ‘non-abiding’ seems quite appropriate. Many Theravada followers, when they hear about non-abiding nirvana have a hard time understanding what it is really about.
Well, non-abiding nirvana is the state of Buddhahood itself. It is the goal that all bodhisattvas, namely those who have made a vow to become a Buddha for the sake of all sentient beings, aspire to. In fact to call it a ‘nirvana’ is not quite correct because it is the state which is neither samsara or nirvana. This is very hard to understand. But we can begin to approach it by looking at the usually perceived distinction between samsara and nirvana. Samsara is the ocean of sufferings. Beings drown in this and try to find solace and resting place in the ocean any way they can, but due to their own ignorance of the very nature of things as empty they keep on deluding themselves and as a result they keep on swimming in the ocean. The arhats are those who have destroyed all the ignorance and defilements that keep them within samsara. So they are, so to speak, ashore. They do not swim in samsara any more. Less metaphorically, they are not born again. They are forever cut off from samsara, residing in the blissful space of static nirvana.
This ‘static’ nirvana is what the Buddha talked about when he first taught to his students, and it is the goal of all Theravana practitioners. However, the Theravadins do not regard this nirvana as ‘static’ because for them there is only one kind of nirvana. In Mahayana, however, there is another kind, the non-abiding, or non-static nirvana, which is the state of attainment of perfect Buddhahood and not the arhats.
The main difference between static and non-abiding nirvana is that those who attain the latter actually speaking reside neither in samsara or nirvana. For them the distinction between samsara and nirvana breaks down completely. The arhats believe that there is such a distinction and they forever remain on the side of the static nirvana. The Buddhas and highly realized Bodhisattvas, on the other hand, do not remain in this static condition, for they are always motivated by their bodhicitta vows to help ferry sentient beings across to the other shore. So they cannot remain completely still and static. They have to move and act. So on the one hand we can say that they are in nirvana (or they have attained nirvana) because they, being Buddhas, have completely destroyed all causes of being compelled by the force of karma, but on the other they do not have to remain in that blissful, static state. As a result they can take up many forms in order to realize their vow. These forms are known as ‘nirmanakaya’ or emanation bodies. One who completely embodies the qualities of a Bodhisattva is an emanation body of that Bodhisattva.
However, being neither in samsara or nirvana, Buddhas and highly realized Bodhisattvas (such as Avalokiteshvara, Tara, Manjushri and others), are free to travel anywhere. They can take up emanation bodies and stay in samsara. They are even there in pure forms within samsara which only highly attained practitioners can directly perceive. So in a real sense the Buddha himself, as well as Avalokiteshvara, Tara and others are now here watching over us.
But don’t take this to mean that they really exist like those deluded beings in samsara are attached to existence. It is not like that at all. Once you see that they really exist, then you are deluded. Everything is empty of their inherent nature, Buddhas and Bodhisattvas not excluded. So they are empty too. So on the one hand they do exist, because they reside neither in samsara or nirvana, and hence can be in either way, but on the other since they are empty they are not there to exist anywhere to begin with. You will get a hang of this after you contemplate on emptiness for quite some time :-)
So how does one know that non-abiding nirvana actually exists? Well, the best we unenlightened beings can do is to use our reasoning faculty. Since nirvana (of either kind) is a state that results from total elimination of all causes of suffering which lead a being to continue to swim in the samsaric ocean, then one who attains nirvana (of either kind) does not have to be born again. But here is the difference between the arhat and the realized Buddha. The arhat does not make the bodhicitta vow, so once he (or she) attains nirvana, then everything is over for him (or her). The Buddha, on the other hand, is moved by the sufferings of all the sentient beings so he (or she) cannot remain still. Realizing that both samsara and nirvana are all empty, the Buddha transcends that distinction and can remain wherever he or she is needed. Hence his state is called “non-abiding.”