there are siddhas, and there are mahasiddhas...
siddha means power, or superknowledge, or even "magic powers", depending on the translators' whim...
there are six mahasiddhas, or "great powers", and they are...
1) awareness of the siddhas and how to cultivate them, (pretty simple to understand, you have to know what they are and some basic steps to cultivate them...)
2) divine sight (like clairvoyance),
3) divine hearing (like clairaudience),
4) knowledge of the minds of all living beings (like telepathy),
5) knowledge of all forms of previous existences of self and others (knowledge of your own previous briths and rebirths and those of others), and
6) power to appear at will at any place (a bit like astral travel or remote viewing)...
how does this relate to buddhism and hinduism..?
well... there is a bodhisattva, a (kind of) buddhist deity, called mahasiddha... he you will encounter if you look into buddhist tantra... in buddhism, the siddhas are related to tantra, and you should look there for answers... In hinduism, the God who is best placed to ask about siddhas is Siva and his consort Shakti, and the siddhas are there in yantra...
the shakti is the source of the power, you see, she is the goddess within you... when you read the bit below about yogi heat, you can also think of Shakti...
buddha himself was supposedly in possession of these siddhas, and his birth name was Siddhartha, or, my translation- (he who) asks about the meaning of (artha) siddha (the superknowledges)...
personally, I prefer the term superknowledges, to power, as there are better sanskrit words for power...
erm... the siddhas are mentioned in the 37 factors to enlightenment too, and are referred to as the pancha-balas, or five (panca) strengths (bala), and while all the siddhas there are just siddhas, there are still only six mahasiddhas...
the siddhas are fruit of the path- they are not the goal of the path, but succulent fruit which ripens along the way, treats to tempt you into maintaining your practise, showing you just what's available to a superior functioning consciousness...
the best way to cultivate the siddhas... maybe taking a LSD taxi will get you there, but once the acid wears off so do the powers...
if the powers wear off like that, can you really say you own them? I don't think so... the most usual starting point is meditation, samatha meditation, cessation meditation, not-thinking meditation...
you sit, you think of nothing... you do this for as long as possible... only when you can do this at will should you start messing about with questing for powers...
as well as being able to concentrate... you need yogi heat... without either of these things you will never have the mahasiddhas...
only when you can enter meditative equipoise and generate the yogi heat should you practise tantra, otherwise all you're doing is fantasising and wasting time.
what is the yogi heat? It's bizzare. It's like... energy, coursing through your body... it's healing, powerful, protective, warming, blissful. Once you stagger upon this heat, you have to learn how to generate it within yourself.
You can find things that feel like it in music, love, beauty, that little tingling sensation when you open your heart to someone or something, that feeling you get when you look at your babies, or your dog... that's like the heat...
you have to be able then, to enter meditative equipoise, and generate the yogi heat... when you can do this, the mahasiddhas will come...