Upvote:0
Probably, the story of the vow was an exaggeration in the Jataka, and seems to hint of extreme asceticism.
Perhaps, the truth behind the actual story might be the Buddha's resolve and effort i.e. Right Resolve (aka Right Intention) and Right Effort. It was more of a resolution and a decision, rather than a vow or a promise.
As you can see in the sutta quote below, too little energy results in laziness. Too much energy results in restlessness.
However, the right amount of energy, along with the rest of the Noble Eightfold Path, will bring the right results.
So, basically, the Buddha resolved to apply the right amount of energy into his effort.
“What do you think, Soṇa? When you were still a layman, weren’t you a good harp player?”
“Yes, sir.”
“When your harp’s strings were tuned too tight, was it resonant and playable?”
“No, sir.”
“When your harp’s strings were tuned too slack, was it resonant and playable?”
“No, sir.”
“But when your harp’s strings were tuned neither too tight nor too slack, but fixed at an even tension, was it resonant and playable?”
“Yes, sir.”
“In the same way, Soṇa, when energy is too forceful it leads to restlessness. When energy is too slack it leads to laziness. So, Soṇa, you should apply yourself to energy and serenity, find a balance of the faculties, and learn the pattern of this situation.”
“Yes, sir,” Soṇa replied.
AN 6.55
Upvote:2
I think it could be seen as a metaphorical gesture as well as a literal event. One might say it is a noble response emerging from the very tethers of samsara, where you have just had enough of the whole thing and are prepared to do anything.
One might also consider that some practitioners can sit within a samadhi state for what seems like an unnatural amount of time with little to no adverse effect on the body. Also, the Juñha Sutta describes an instance where sariputra, whilst in deep meditation, is hit over the head very hard by a yakkha, but having emerged from his meditation sariputra only reported a slight headache. The Buddha, having saw what occurred with his divine eye, said this...
Whose mind, standing like rock, doesn't shake, dispassionate for things that spark passion, unprovoked by things that spark provocation: When one's mind is developed like this, from where can there come to him suffering & stress?
So then, resolving to sit under the Bodi tree gave him this understanding which he then shared in various creative ways through the medium of wisdom bullets.
Upvote:4
The story of the vow is from the Jhataka (see here).
The story is impressive and much-loved, but maybe the Jataka is a slightly different category than the other suttas (see also this answer). The version in the suttas e.g. MN 26 doesn't include that detail.
As an aside, since you're interested in Zen, I wonder if the story has any parallels with Zen story Three Days More.