Always chamber ammunition (live, dummy, snap caps) directly from the magazine whenever you can. Avoid manually placing a cartridge in the chamber and then closing the slide.
When the cartridge is fed from the magazine the case rim slides behind the extractor as the cartridge rides up the breechface.
When the cartridge is manually placed in the chamber and the slide closed the extractor must spring around the case rim to engage the extractor groove. Extractors are designed to do this (witness the beveled pawl-like edge) but some extractor designs are not as sturdy as others and can be damaged by the practice, hence the general rule not to do it with any pistol.
Sometimes you cannot help doing this - for example, when you insert a failure to extract stoppage for training. You'll manually place a cartridge or case into the chamber, seat a magazine, and then put the slide in battery to jam the top cartridge of the magazine into the case head of the cartridge/case in the chamber. When the immediate action of Tap, Roll & Rack fails to get the pistol running, you perform a Combat Reload. You won't be able to insert the fresh magazine because the "spent" magazine didn't jettison from the magazine well. When you cannot insert the fresh magazine you immediately stow it between the ring and pinky fingers of your firing hand, to clear your support hand to operate the slide. You'll smartly retract and lock the slide, press the magazine release and forcibly remove the "spent" magazine from the magazine well. When the "spent" magazine is removed you'll Roll the pistol to point the ejection port at the ground and smartly retract and release (rack) the slide three times to clear the action. The first rack engages the extractor with the extractor groove of the cartridge/case in the chamber, the second rack *should* extract and eject it, and the third rack is positive measure to ensure the chamber is clear. After you've racked the slide three times, you'll complete the Combat Reload by retrieving the fresh magazine from between your ring and pinky fingers, insert and seat (Tap). Then you'll Roll & Rack to load the chamber.