Shabbat Hotplate

Question

I recently purchased the Topheat Shabbat hotplate. From what I understand, given that it has no knobs to adjust the heat & that the heat is below 120F, one can place cold cooked food on it on Shabbat. This presupposes that the unit is operating. What happens, if I use a timer? Specifically, if I set the timer to go on Shabbat morning, when should the cold food be placed on the warming plate? Does it make a difference whether the food is placed on before or after the timer turns the unit on. Based on some of the previous postings, I am somewhat confused. Your explanation and recommendations on the proper use of the Topheat warming plate would be appreciated. Thanking you in advance.