Of all evil not do,
of all good practise,
and to self-purify this mind,
these are all Buddhas’ teachings.
— Śākyamuni Buddha
(Dharmapada: Verse 183)
As hell-beings (地狱众生), especially those in Uninterrupted Hells (无间地狱), have constant suffering, how do Bodhisattvas (菩萨) help to free them from suffering? As suffering arises with the combination of past karmic causes (过去业因) and present supporting conditions (现在助缘), when conditions are changed, karmic causes will not be able to ripen (fully) in the moment.
Understanding this, Bodhisattvas and even manifested Buddhas will use all kinds of skilful means to encourage hell-beings’ conditions to be changed, by encouraging changing of their thoughts, words and/or deeds. This can be done in the following ways. (These are also ways to avoid falling into any hell in the place.)
 Repentance (忏悔): By inspiring sincere repentance with genuine remorse, perhaps by reminding them of their past misdeeds done, whenever karmically possible (as even hell-beings usually still have some distant past good karmic causes), perhaps by manifesting mental flashbacks to spur moral shame. This practice reflects this aspect of all Buddhas’ teachings (诸佛教), at least in part – ‘of all evil, not do’ (诸恶莫作).
 Doing Good (行善): By inspiring thinking, speaking and doing of good, whenever karmically possible, perhaps by manifesting as other suffering hell-beings asking for help or needing help. Even if physical helping is difficult, when seeing others also suffering or suffering more, at least having the wish to help them, to share or take on their suffering. This practice reflects this aspect of all Buddhas’ teachings, at least in part – ‘of all good, practise’ (众善奉行).
 Mindfulness Of Buddha Or Bodhisattva (念佛菩萨): By inspiring sincere mindfulness of Buddha with the name of Āmítuófó (阿弥陀佛) to be reborn in his Pure Land (净土), whenever karmically possible, perhaps by manifesting as other hell-beings practising it successfully. This is the most powerful and effective method for exiting hells instantly, although many hell-beings, being very evil, might lack strong karmic affinity with Āmítuófó.
If there is affinity with say, a certain Bodhisattva, sincere mindfulness of him or her can alleviate suffering in the moment too, as this creates good or pure karma, that weakens evil karma’s fruition, even if there is yet to be Aspiration (愿) to reach Pure Land. This practice reflects this aspect of all Buddhas’ teachings, at least in part – ‘to self-purify this mind’ (自净其意).
When hell-beings’ change their present conditions, in thought, word an/or deed, as challenging as this might be, their past evil karma’s fruition is thus mitigated or eliminated. For hell-beings with little good karma, which makes Bodhisattvas’ and Buddhas’ direct and strong assistance more difficult, there will be need to have a gradual snowballing effect over some time.
That is, little by little, with more avoiding of evil, more doing of good, and/or more purifying of their minds, thus creating new good and/or pure karma, more and more of their hellish conditions can be changed, to eventually no longer deserve stay, thus leaving the hells for better rebirths.
With their perfect and inexhaustible compassion, all Bodhisattvas and Buddhas are always doing their best possible to guide all beings, including hell-beings, towards (Pure Land for) Buddhahood. Thus, the onus is upon us to heed their great teachings and efforts, so as to not let them (and ourselves) down.
At once mindful of Buddha,
end evil and practise good,
and with Self and Other-power,
completely purify this mind.
One Hell Of An Act Of Compassion