Trivia: The Alien Design for Sil was done by H.R Giger, the same man who designed the Xenomorph in the Alien movies.
Trivia: Creature designer H.R. Giger personally financed the "nightmare train" sequence after the studio refused to fund it due to its high cost and short length. It reportedly cost him nearly $100,000 between paying for the props, the filmstock, the effects and the camera crew.
Trivia: Creature designer H.R. Giger reportedly came up with the way Sil is killed off at the end. In the original script, Sil is merely set on fire with a flamethrower. Giger told the studio the ending was "weak," and suggested that her head should get blown off, and then her body should be burned up, leading to the ending seen in the finished film. Giger also suggested a more elaborate final battle set in a drive-in movie theater with members of the military storming the location, but it went unused for tone and budgetary reasons.
Trivia: Young Sil is played by future movie-star Michelle Williams. However, she reportedly dislikes the film because she was terribly bullied by other children about being in it after it was released. It brings up too many bad memories for her.
Trivia: Sil's alien sound effects were primarily provided by Frank Welker. Welker is a well-respected voice-actor with over 800 credits to his name beginning in the late 1960's. He also specializes in animal and creature sound effects.
Answer: You could probably make a compelling argument that, as an alien-hybrid, Sil is stronger than a normal person and thus was just able to forcibly cut through the finger with the pruners by "crushing" it. Also, this is where good old suspension of disbelief comes in. The pruners might realistically have broken at some point, but it's a movie, so we can "forgive" this bit of unrealism because it creates a compelling scene.
TedStixon