The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. They say: 'What is the meaning of preṅkha, swing?' Verily, he is the swing, who blows (the wind). He indeed goes forward (pra + iṅkhate) in these worlds, and that is why the swing is called preṅkha.
2. Some say, that there should be one plank, because the wind blows in one way, and it should be like the wind.
3. That is not to be regarded.
4. Some say, there should be three planks, because there are these three threefold worlds, and it should be like them.
5. That is not to be regarded.
6. Let there be two, for these two worlds (the earth and heaven) are seen as if most real, while the ether (space) between the two is the sky (antariksha). Therefore let there be two planks.
7. Let them be made of Udumbara wood. Verily, the Udumbara tree is sap and eatable food, and thus it serves to obtain sap and eatable food.
8. Let them be elevated in the middle (between the earth and the cross-beam). Food, if placed in the middle, delights man, and thus he places the sacrificer in the middle of eatable food.
9. There are two kinds of rope, twisted towards the right and twisted towards the left. The right ropes serve for some animals, the left ropes for others. If there are both kinds of rope, they serve for the attainment of both kinds of cattle.
10. Let them be made of Darbha (Kusa grass), for among plants Darbha is free from evil, therefore they should be made of Darbha grass.
172:3 After having considered the Marutvatîya, he proceeds to consider the Nishkevalya. This has to be recited by the Hotri while sitting on a swing.