Gridview Findcontrol

I'm using the following code to find a textbox:

txtM1Amt = e.Row.Controls(4).FindControl("txtM1Amt_add")

I inherited this application and I'm just curious as to how it works.  I follow that the row has a controls collection but I don't understand what controls are included.  If the user clicks "Edit" on this row and it brings up the update and cancel buttons, how does that affect the count of the controls.  I have this same code in the footer row and it works fine, not the case in the actual grid.

Thanks in advance,

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