illustration (attribution, if any possible, is at the end of the article)
Karma remains one of the difficult topics in Buddhism. This is largely due to the misconceptions that are already abundant, and which need to be addressed before causality may be explained.
For example, 'causality' is quite fundamentally different from the idea of 'justice': facing the consequences that tend to arise from particular behaviours isn't the same as facing a 'superior' judgement on the said behaviours.
For example, 'combination' of factors is quite fundamentally different from the idea of 'accumulation'…
Things are even more muddled by the adaptation of teachings to (wholesomely) surf the ignorance of the crowds.
For example, Theravāda Buddhism includes in its teachings a transfer of merits. A priori, this would seriously support the notion that wholesome karmic seeds can not only be accumulated but can even be transferred!
But this is solely a teaching to support lay beginners on the Path: it supports a practice used to embody generosity, to embody compassion, to embody loving-kindness… It is a practice easily misconceived in philosophical terms (about what karma is or isn't) but it is a pedagogical practice meant to guide the practitioners on a path of actualising wholesome qualities! Since all 'views' have to be relinquished to attain Nirvāṇa anyway, it doesn't really matter if you misconceive karma at the start of your cultivation… as long as the misconception guides you to wholesome tendencies!
Theoretically-correct 'karmic' explanation of "transfer of merit"
In buddhist terms, when you 'pray' for the benefit of others, you might simply wish for their difficulties to come to their 'natural' end (since all conditioned phenomena are impermanent) without creating too much suffering in the meantime.
It relies on knowing that the suffering comes in large part from the mental attitude of the sufferer rather than the circumstances.
You're not asking an external god to intervene, you count on causality to play out naturally without it being 'experienced' as 'suffering'.
In a way, you pray to the buddha-nature in the sufferer, i.e. you call for the sufferer himself/herself (not an external entity) to manifest wisdom at this point in time, in order to ride a phenomenon without giving into the mental fabrications which would lead to suffering.
This is the principle behind the "transfer of merit" in Theravāda.
Karma is fundamentally a causal law, it is as impersonal as the law of gravity might be. However, karma is 'manifested' mostly at the individual level (just like gravity is manifested by its action on specific objects, not just empty space) so we might say that karma is 'personal' (and many sūtras describe it at such "conventional truth" level).
When karma is seen as personal, this is based on an individual 'appropriation' (gplus.wallez.name/KZRPUvkHpLg) rather than inherent individualism (rejected by selflessness).
This creates a difficulty, in that you cannot just 'make' someone ignorantly clinging to an appropriation switch to another (even if you were the Buddha, you cannot): their ignorance make them believe that releasing the grasp from their current appropriated causal narrative would mean death of the 'self'… hence their ignorance is seriously leading them not to let go, in a self-fulfilling 'cyclical' saṃsāra!
Even when practitioners say they want to let go of biases, tendencies, preferences, karma, etc., they tend to want so only in a comfortable manner: they often seek the "saṃsāra with benefits", the letting go of everything except the 'self' (kept to enjoy the benefits)!
You cannot change (by transfer or otherwise) the karma of someone else: only they can, because only they ignorantly 'pick' which narrative they embody a clinging to!
So you cannot really transfer 'good' karma to someone else, but the Theravāda school says there is a possibility somehow equivalent to transfer, in 3 steps:
• (Even if knowing it is not possible,) Alice may truly, deeply wish she could transfer good karma for the benefit of Bob;
• This is a wholesome wish, a good intention, which in itself creates good karma (habit of selfless loving-kindness, compassion, care) for Alice, even if the wish cannot be truly fulfilled in the present context;
• If Bob is capable of rejoicing (out of compassion or loving-kindness for Alice) about the good karma now created by Alice, then Bob himself is manifesting a wholesome intention, hence creating good karma himself!
In fact, there was no transfer (as in: a zero-sum win-loss), but there was creation of good karma for both involved… Hence, in some way, there was transfer, it 'worked': the intention of Alice did create a 'context' in which Bob's karma was improved!
Photo: prayer at Tokyo's Zojoji buddhist temple (31/12/2008), unattributed