四分律刪繁補闕行事鈔 2.324

2.324

不挑鉢中者，置四邊挑鉢中央至鉢底。不犯者：有病、患熱開中央令冷、日過、難緣者開。

Not digging into the middle of the bowl means leaving the four sides and digging into the center of the bowl down to the bottom. One does not commit an offense if: being ill, suffering from heat and opening the center to cool it, the time expiring, or under perilous circumstances, it is permitted.