Mi Sheyirtzah - Whoever wants

The Mekor Chaim* (137) enumerates the Chiyuvim to be Oleh Latorah according to Minhagei Vermeize (Worms).

Amongst the Chiyuvim is Mi Sheyirtza (Whoever wants). 
His Chiyuv for an Aliyah is on the first day of Shavouos and on the first day of Rosh Hashana.

1) How do you become a Chiyuv just because you want ?
2) Why on the first days of Shavouos and Rosh Hashana ?


There are different Minhagim whom & how to call up for the
Aliyah of the Tochocho (admonition).

In many Ashkenazi Shuls they still have the old Minhag not to call up for the Tochocho Aliya by name, instead they call him up,

“Ya’amod Mi Sheyirtzah”.

Some Shuls, it was the Shamash*  who went up for the Aliyah. Other places there was a designated person who volunteered to take this Aliyah.

To compensate for taking the Tochocho Aliya , he was given an Aliyah on the first
Yom Tov after the Tochocho. (Bechukosai – Shavuos , Ki Savo – Rosh Hashana)

*R.Yair Chaim Bachrach (Chavas Yair) 

* The Mahril writes that part of the contract the Shul made with the
Shamosh , was that he was obligated to be Oileh the Tochocho