Which function trigger to load next set of data when user scroll datagridview c#

I am working with datagridview. My datagridview virtual mode is turned on.so i populate grid this below way

privatevoid dataGridView1_CellValueNeeded(object sender,DataGridViewCellValueEventArgs e){
    e.Value= memoryCache.RetrieveElement(e.RowIndex, e.ColumnIndex);}

basically i took the code from a msdn link and customize bit. one thing is not clear to me which function is getting called first when user scroll datagridview. please tell me the function name.

also tell me how relation has been established between datagridview scroll and function call to load next set of data. here i am not posting my full code rather here leaving a url from where anyone can see the full working code.

here is the url http://stackoverflow.com/questions/31458197/how-to-sort-datagridview-data-when-virtual-mode-enable/31475803#31475803

please have a look and help me to understand the area which is getting called when user scroll down thedatagridview.

thanks