It fires by pulling the trigger like virtually all guns made in the past 700+ years.
From the part of the article I didn't post:
"It’s an elite handgun intended for law enforcement and military personnel who may need to fire it with split second notice. Hence, it has a hair trigger in single-action mode. Even among well-trained users, it has a lengthy history of accidental discharges.
..."The SIG Sauer in Lopez Sanchez’s case has three features prone to accidents: 1. No safety lever, making it perpetually ready for firing. 2. Manufacturer-issued trigger pull of 4.4 pounds of force (in single-action mode), which is among the lightest on the market. 3. An unlabeled decocking lever despite being essential to disengage the single-action mode. (The SIG Sauer safety manual urges “DO NOT THUMB THE HAMMER DOWN the consequences can be serious injury or death — only and ALWAYS use the decocking lever.”)"