1Thus have I heard. On one occasion the Blessed One was dwelling at Sāvatthī in Jeta's Grove, Anāthapiṇḍika's Park. Then, when the night had advanced, a certain devata of stunning beauty, illuminating the entire Jeta's Grove, approached the Blessed One. Having approached, he paid homage to the Blessed One, stood to one side, and said to him:

1Evaṁ me sutaṁ—​ ekaṁ samayaṁ bhagavā sāvatthiyaṁ vihārati jetavane anāthapiṇḍikassa ārāme. Atha kho aññatarā devatā abhikkantāya rattiyā abhikkantavaṇṇā kevalakappaṁ jetavanaṁ obhāsetvā yena bhagavā tenupasaṅkami; upasaṅkamitvā bhagavantaṁ abhivādetvā ekamantaṁ aṭṭhāsi. Ekamantaṁ ṭhitā kho sā devatā bhagavantaṁ etadavoca: 

"How, dear sir, did you cross the flood?"[n.1] Mārisa, "dear sir," is the term which the devas generally use to address the Buddha, eminent bhikkhus (e.g. see the Pali for SN40:10), and members of their own community (SN11.3); kings also use it to address one another (SN3.12). Spk explains it as a term of affection meaning "one without suffering" (niddukkha), but it is probably a Middle Indic form of Skt madṛsa. The word "flood" (ogha) is used metaphorically, but here with technical overtones, to designate a doctrinal set of four floods (see SN45.171), so called, according to Spk, "because they keep beings submerged within the round of existence and do not allow them to rise up to higher states and to Nibbāna." The four (with definitions from Spk) are: (i) the flood of sensuality (kāmogha) = desire and lust for the five cords of sensual pleasure (agreeable forms, sounds, etc.—see SN45.176); (ii) the flood of existence (bhavogha) = desire and lust for form-sphere existence and formless-sphere existence and attachment to jhāna; (iii) theflood of views (diṭṭhogha) = the sixty-two views (DN I); and (iv) the flood of ignorance (avijjogha) = lack of knowledge regarding the Four Noble Truths.

"Kathaṁ nu tvaṁ, mārisa, oghamatarī"ti?

"By not halting, friend, and by not straining I crossed the flood."[n.2] Appatiṭṭhaṁ khvahaṁ āvuso anayūhaṁ ogham atariṁ. Spk: The Buddha’s reply is intended to be paradoxical, for one normally crosses a flood by halting in places that offer a foothold and by straining in places that must be crossed.
Spk glosses appatiṭṭhaṁ only with appatiṭṭhahanto (an alternative form of the present participle), but Spk-pṭ elaborates: "Not halting: not coming to a standstill on account of the defilements and so forth; the meaning is ‘not sinking’ (appatiṭṭhahanto ti kilesādīnaṁ vasena asantiṭṭhanto, asaṁsīdanto ti attho)." The verb patitiṭṭhati usually means "to become established," i.e., attached, principally on account of craving and other defilements. Consciousness driven by craving is "established" (see SN12.38–40, SN12.12,SN22.53–54), and when craving is removed it becomes "unestablished, unsupported." The arahant expires "with consciousness unestablished" (appatiṭṭhitena viññāṇena … parinibbuto; see SN4.23). All these nuances resonate in the Buddha’s reply.
The verb ayūhati is rare in the Nikāyas, but see below v. 263df, v. 264d, and Sn 210d. It is an intensification of ūhati (augmented by a- with -y- as liaison); the simple verb occurs at MN19 verses 6, 7 and 15, where it might be rendered "to be strained." Its occurrence there ties up with the present context: a strained mind is far from concentration. In the later literature the noun form ayūhana acquires the technical sense of "accumulation," with specific reference to kamma; in the formula of dependent origination (paṭiccasamuppāda), volitional formations (saṅkhāra) are said to have the function of ayūhana; see Paṭis I 52,14, 26; Vism 528,12 (Ppn 17:51), 579,31–580,4 (Ppn 17:292–93).
Spk: The Blessed One deliberately gave an obscure reply to the deva in order to humble him, for he was stiff with conceit yet imagined himself wise. Realizing that the deva would not be able to penetrate the teaching unless he first changed his attitude, the Buddha intended to perplex him and thereby curb his pride. At that point, humbled, the deva would ask for clarification and the Buddha would explain in such a way that he could understand.

"Appatiṭṭhaṁ khvāhaṁ, āvuso, anāyūhaṁ oghamatarin"ti.

"But how is it, dear sir, that by not halting and by not straining you crossed the flood?"

"Yathākathaṁ pana tvaṁ, mārisa, appatiṭṭhaṁ anāyūhaṁ oghamatarī"ti?

"When I came to a standstill, friend, then I sank; but when I struggled, then I got swept away. It is in this way, friend, that by not halting and by not straining I crossed the flood."[n.3] The Buddha’s brief reply points to the middle way (majjhimā paṭipadā) in its most comprehensive raṅge, both practical and philosophical. To make this implication clear Spk enumerates seven dyads: (i) "halting" by way of defilements, one sinks; "straining" by way of volitional formations, one gets swept away; (ii) by way of craving and views, one sinks; by way of the other defilements, one gets swept away; (iii) by way of craving, one sinks; by way of views, one gets swept away; (iv) by way of the eternalist view, one sinks; by way of the annihilationist view, one gets swept away (see It 43,12–44,4); (v) by way of slackness one sinks, by way of restlessness one gets swept away; (vi) by way of devotion to sensual pleasures one sinks, by way of devotion to self-mortification one gets swept away; (vii) by way of all unwholesome volitional formations one sinks, by way of all mundane wholesome volitional formations one gets swept away. ñāṇananda suggests connecting the principle of "not halting, not straining" with each of the four floods: see SN-Anth 2:56–58.

The devata:

"Yadāsvāhaṁ, āvuso, santiṭṭhāmi tadāssu saṁsīdāmi; yadāsvāhaṁ, āvuso, āyūhāmi tadāssu nibbuyhāmi. Evaṁ khvāhaṁ, āvuso, appatiṭṭhaṁ anāyūhaṁ oghamatarin"ti.

2v.1 "After a long time at last I see
A brahmin who is fully quenched,
Who by not halting, not straining,
Has crossed over attachment to the world."[n.4] Spk: The Buddha is called a brahmin in the sense of arahant (see Dhp 388, 396–423). He is fully quenched (parinibbuto) in that he is quenched through the quenching of defilements (kilesanibbanena nibbutaṁ). Craving is designated attachment (visattika) because it clings and adheres to a variety of sense objects.

2"Cirassaṁ vata passāmi,
brāhmaṇaṁ parinibbutaṁ;
Appatiṭṭhaṁ anāyūhaṁ,
tiṇṇaṁ loke visattikan"ti.

3This is what that devata said.[n.5] Spk: When the deva heard the Buddha’s reply he was established in the fruit of stream-entry. The Teacher approved. Then that devata, thinking, "The Teacher has approved of me," paid homage to the Blessed One and, keeping him on the right, disappeared right there. sn.i.2

3Idamavoca sā devatā. Samanuñño satthā ahosi. Atha kho sā devatā: "samanuñño me satthā"ti bhagavantaṁ abhivādetvā padakkhiṇaṁ katvā tatthevantaradhāyīti.