The Shulhan Aruch rules that one must be careful when Koshering utensils with Ha’agalah that the vessels do not touch each other, so that the boiling water can have direct contact with all surfaces to extract the Hames. Therefore, one must avoid, putting in too many vessels at one time. If he is using a tong to hold the vessel, he must loosen the grip of the tong so that the waters can touch the spot where he is holding it. Similarly, if he is using a net, he must shake it, so that the waters will penetrate all sides.
Maran also brings the Halacha that one may not do Ha’agalah with the hot springs of Tiberias, because their heat source is the sun, whereas the Hames became absorbed in the pot through the heat of fire. In order to extract, the heat source of the Ha’agalah must be the same as the cooking.
According to Maran, glass vessels do not absorb, and therefore never require Ha’agalah. This includes Pyrex and Duralex, as well. Ashkenazim have a different Halacha and hold that glass is in the same category as earthenware vessels, which can never be koshered. Thus, there are two extreme positions: Sephardim hold glass is non-porous, whereas Ashkenazim hold it is ultra-porous.
Maran brings the custom from the Rambam that after the Ha’agalah the vessel is soaked in cold water. According to the Mishna Berura, the reason is to prevent the hot water with the Hames extract from being re-absorbed in the vessel. If one did not do so, it is still kosher. One should be careful with certain vessels that may crack if transferred immediately from hot to cold.