Hello Snake,
What you are asking is a complex question that cannot be answered with just a few lines, but I’ll give a more complete answer here.
To say a full body equals momentum is not entirely true nor accurate. It can = momentum, and it cannot. How?
Well, if we are in an uptrend, and we have a full body bear candle, that would not equal momentum for the uptrend. Depending upon the size of the full body candle, it could equal profit taking (if its small), it could equal a huge amount of selling and opposition (if its a large candle), it could me massive profit taking and the end of the trend, or it may mean a reaction to a news event.
So as you can see, just from this one example, there is no way to fully answer your question and give you a be-all-end-all answer to what a full body candle means because you always have to take it into context. A single bar by itself will mean only so much, but taken into context, it will mean a lot more.
Long tails are generally rejections, but a better way to explain them would be, they are a rejection of price at a particular level where the rejection took place. Now, by itself, a long wick or rejection could be a serious threat to change the current trend or swing, or not. Again, you have to take it into context. What if you are looking at a candle on the 5min chart and you see a long tailed rejection? It may mean on an intraday basis there are orders rejecting at that level, but the rejection and move may be short lived, and the uptrend may resume.
Again, it all comes down to context.
So what you are asking is a loaded question that needs to be fully unpacked over time and also needs a lot of pretext for as well.
But momentum for a swing or move is rarely ever fully defined by one candle, and usually is a series of candles, like exhaustion bars, climax price action, etc, engulfing bars, etc.
But hopefully this gives you a greater idea. The key is to understand how to read price action in real time, and how to understand the order flow behind it and that creates it.
This is not something that can be learned or explained in one post, but over time.
So the good thing is you are asking a very poignant question which means you are onto something and curious about the right thing. The downside is your answer requires unpacking, more material and time to understand fully as there are many ways to interpret momentum and candles with long wicks or or full bodies.
I do recommend starting with learning how to read the impulsive vs. corrective price action and that will be a tremendous guide to understanding momentum, price action and order flow.
I hope this helps give you a better picture of whats going on.
Kind Regards,
Chris