The power is in the breath. So Thoth said.
The humming breath is the foundation of mantra. It is a basic pranayama you can find on the list of breathing exercises on ToZ. The more drawn-out the breath is (WITHOUT STRAINING YOURSELF!)*, the more powerful is your mantra practice. Intent and focus can make up part of this, as some people who are not really ready to use mantra tend to get distracted when vibrating properly. However, in no way you can achieve with fast vibrations as much power as proper vibration done by someone is actually read for mantra (laser focus, full-on intent).
There was a scheme HP HC shared a while ago. I don't remember which topic was this in. It would be great if someone could link it, in order to offer a fuller picture of the HP's post. It went like this, from the most powerful to the least:
:arrow: 1. Full vibrations
:arrow: 2. Fast vibrations
:arrow: 3. Whispered vibrations
:arrow: 4. Mental vibrations
Obviously, the vibration should be of the proper frequency for what you're working, and you will feel it in the specific part of the soul. If it's not the right frequency, you're doing little to nothing at all.
*Caps are necessary in this case, because I had some people in the past who needs it like, or they're going to start verbal fights just for the sake of arguing with me.
Do I have to hold the breath for a while and then vibrate, or I can just vibrate after breathing in? I basically just vibrate without holding. But in the opening chakras, it is required to hold breath after breathing in. What is the purpose of holding breath?
It has to do with your focus, absorbing energy doing the breath and also the vibration of the chakra, which is better to complete the cycle... let me try to explain.
If you for example, have a glass or a tuning fork, if you hit them with a metal rod, they will vibrate with their own frequency of resonance, and you will hear their harmonic sound (their frequency of resonance)
Now, if you hit them again, while they still vibrate loudly, there are 2 cases which can happen,
You hit the tuning fork once, so it vibrates with the frequency of resonance, now while it is vibrating still strongly, you attempt to hit it again...
1: (which will be almost always the case)
You don’t time it perfectly, and you hit the tuning fork, which will dampen the sound, and also bring the tuning fork out of it‘s natural frequency of resonance.
2: you hit perfectly, you amplify the sound. (Which virtually never happens perfectly as you can not time this)
So the best way would be, to hit the tuning fork, let it vibrate, and before you hit it, it needs to lose energy on it‘s own, after is either completely still, or significantly quieter, you can hit the tuning fork again, and you get another harmonic sound with its own frequency of resonance.
Hit *harmonic sound* - wait *it gets quieter over time as it vibrates* - hit * harmonic sound* - wait.... and so on.
You can not spam hit a tuning fork because this will not give you strong and harmonic sounds, which is what the OP does.