When in the down (safe) position, the safety lever rotates a drum within the slide, which blocks the firing pin by means of narrow shoulders on opposite sides of the firing pin. If the hammer is cocked when the safety is moved downward, the hammer will fall. Unless restrained and carefully lowered, the hammer will then hit the firing pin, which will slam into the small shoulders on the safety drum.
Because of the narrow engagement surfaces, the shoulders may wear and allow the firing pin to travel forward and fire the chambered round. That will then cause the gun to cycle, and because of the automatic hammer-drop, it will continue to do so until the magazine is empty. Such sudden and unexpected machine-gun firing of your pistol is, of course, unsafe as well as unsettling.
For that reason, the hammer should not be allowed to fly forward when decocked. As the old Walther booklet explains, the hammer should be gently lowered after decocking, while keeping the gun pointed in a safe direction.
In his book "Troubleshooting Your Handgun," noted handgun authority J.B. Wood goes so far as to recommend having a gunsmith remove the hammer-drop lever (called the "relief piece" by Walther) from your P38, and using both hands to lower the hammer in order "to be doubly safe." Note: this may not be as true of commercial models as it is of military arms, but I still recommend manually lowering the hammer when decocking.
As with any gun, no matter how many safety features it has, never point it at anything you're not willing to shoot.