Since this is the Discovering Theravada forum and some people may not know the simile in question, here is the sutta from which it is derived.
SN 47.19 wrote:On a certain occasion, the Exalted One was dwelling in the Sumbha country, in a township of the Sumbhas, called Sedaka. There the Exalted One addressed the monks:
"Once upon a time, monks, a bamboo-acrobat set up his pole and called to his pupil, Medakathaalika, saying: 'Come, my lad, Medakathaalika, climb the pole and stand on my shoulders!'
"'All right, master,' replied the pupil to the bamboo-acrobat, climbed the pole and stood on his master's shoulder. Then, monks, the bamboo-acrobat said to his pupil: 'Now, Medakathaalika, my lad, you protect me well and I shall protect you. Thus warded and watched by each other, we will show our tricks, get a good fee and come down safe from the bamboo-pole.'
"At these words Medakathaalika the pupil said to the bamboo-acrobat: 'No, no! That won't do, master! You look after yourself, master, and I'll look after myself. Thus warded and watched each by himself, we'll show our tricks, get a good fee and come down safe from the bamboo-pole.'
"Therein that is the right way," — said the Exalted One. "Just as Medakathaalika the pupil said to his master: 'I'll protect myself': so, monks, should the Foundations of Mindfulness be practiced. 'I'll protect others': so should the Foundations of Mindfulness be practiced. Protecting oneself, monks, one protects others; protecting others, one protects oneself. 
"And how, monks, does one, in protecting oneself, protect others? By frequent practice, development and making-much-of (the Foundations of Mindfulness). Thus, monks, in protecting oneself one protects others.
"And how, monks, does one, in protecting others, protect oneself? By forbearance, by non-violence, by loving-kindness, by compassion. Thus, monks, in protecting others, one protects oneself.
"'I shall protect myself': with this intention, monks, the Foundations of Mindfulness should be practiced. 'I shall protect others': with this intention the Foundations of Mindfulness should be practiced. Protecting oneself, one protects others: protecting others, one protects oneself."
Footnotes by venerable Nanananda...
It is noteworthy that the parable in this sutta has some peculiarity in that it is not on all fours with the doctrinal points discussed in relation to it. The maxims presented in connection with the practice of Mindfulness ("I'll protect myself"; "I'll protect others,") are an improvement on that recommended by the acrobat's pupil ('You look after yourself, master, and I'll look after myself"). This is the significance of the Buddha's remark: "Therein, that is the right way." This point seems to have been overlooked when the P.T.S. edition and translation attribute these words to the acrobat's pupil, breaking up and distributing the sentence between two paragraphs (The sentence should read:'So tattha ~naayoti bhagavaa avoca, yathaa medakathaalika antevaasii aacariya.m avoca'). The sentence thus wrongly broken up, is then taken to mean that the Buddha here recommends the same acrobatic principle to the monks. ('... Then said the Exalted One: "Now, monks, just as Medhakathaalika, the pupil said to his master, "I'll look after myself," so ought ye to observe the station of mindfulness...' etc.) That principle, striking as it is, is less broad-based than the twin-principle recommended by the Buddha himself: "Protecting oneself, one protects others; protecting others, one protects oneself." As clearly expounded in the Ambala.t.thika Raahulovaada sutta (M. I 415ff), the way to purify one's bodily, verbal and mental actions is by constant reflection on their repercussions on oneself as well as on others. Mindfulness, then, is that benign agent of transmutation which preserves the inner consistency and harmony of this twin-principle.
78. '...in protecting oneself one protects others': The principle indicated here in brief can be appreciated the better with the aid of the following exhortation by the Buddha at S. II 29:
"Wherefore, monks, you stir up energy that you may reach what is still unreached, that you may attain what is still unattained, that you may realize what is still unrealized. 'Thus will this going-forth of ours not be barren, but fruitful and of consequence. And those offerings of them whose requisites of robes, almsfood, lodgings and medicaments we enjoy, shall, on our part, be of great fruit, of great consequence for them.' Verily, it is thus, monks, that you should train yourselves. For one who discerns his own good, this is enough to call up diligent effort. For one who discerns another's good, this is enough to call up diligent effort. For one who discerns the good of both, this is enough to call up diligent effort."
'The frequent practice, development and making much of mindfulness' recommended by our sutta, is one that is conducive to the good of both oneself and others. As the commentary observes, even the mere appreciation of a monk who, by his diligent practice, attains to arahantship, will be a thought productive of great merit. Besides, one's devotion to the practice and exemplary life can be a source of inspiration to others. Since greed, hatred and delusion are the mainsprings of all evil intentions resulting in harm to oneself and others, in protecting one's mind from them, one is at the same time, protecting others as well.
79. '...in protecting others, one protects oneself:'
Forbearance, non-violence, loving-kindness and compassion, being positive altruistic attitudes, directly concern one's relations with the outside world. Yet, on the mental side too, they exercise a wholesome influence conducive to one's own spiritual growth. They are all 'object-lessons' in the practice of mindfulness.