Makkos, Chapter 3, Mishnah 13. The Mishnah continues to present the details of how the lashes are administered, and it opens with a description of the strap with which the lashes are given.
The details of the strap:
"Yad tefach" - the handle of the strap is a tefach long.
"Rechavah tefach" - the strap itself is a tefach wide.
"V'rosho magaas al pi kereiso" - the end of the strap is designed to be long enough to reach the belly of the one receiving the lashes.
The Gemara points out that since people differ from one another in size, the strap necessarily had to be adjustable - it contained a component that allowed it to be lengthened or shortened according to the size of the person.
The division of the lashes:
"U'makeh oso shalish milfanav u'shtei yados me'achorav" - as was mentioned in the previous Mishnah, a third of the lashes are given on the front of the body, and two thirds behind him, on his back, on the two sides of the back.
The position of the one receiving the lashes:
"V'eino makeh oso lo omeid v'lo yosheiv ela muteh" - the lashes are not administered while the person is standing, nor while he is sitting, but only while he is leaning and bent over. "Shene'emar: v'hipilo hashofeit" - the judge must have him lowered down, and from here we learn that he is in a position of falling; this position is at roughly a forty five degree angle.
"V'hamakeh makeh b'yado achas b'chol kocho" - the one who actually administers the lashes strikes with one hand, but he uses the full strength of that hand.
In summary: in this Mishnah we learned the details of the strap - a handle a tefach long, a width of a tefach, and an end that reaches the belly of the one receiving the lashes, with the strap being adjustable according to the size of the person; the division of the lashes into a third in front of him and two thirds behind him; the requirement that he be bent over, which is derived from the verse "v'hipilo hashofeit"; and the manner of striking - with one hand and with full strength.