The way you tighten a canning jar lid is important to sealing success. If it’s too loose, the jar won’t seal. Too tight, and the lid can buckle or the jar might even break.
During canning, air in the headspace is driven out, forming a vacuum seal as the jar cools. When lids are too tight, the air cannot easily escape and forces its way out by buckling the lid.
New lids can be heated in simmering, not boiling, water but it is not necessary nowadays. If overheated, the rubber compound on new lids becomes very soft; over-tightening might damage or cut through it.
To prevent seal failures, tighten the band “fingertip tight.” Just at the point the band catches on the jar, turn about one more inch and then stop.