ISTP People of this type tend to be: logical, pragmatic, and matter of fact; quiet, unassuming, and autonomous; realistic, pragmatic, and aloof; impulsive and curious about the physical world; flexible and resourceful; objective and unemotional. The most important thing to ISTPs is the freedom to act independently and follow their impulses.
How to Spot ISTPs: quiet, independent, and active
detached, unemotional and calm
comfortable, functional dressers
curious, realistic, and analytical
Tips for Communicating with ISTPs: Avoid emotional appeals - use clear logic
Respect their privacy and never force intimacy
Listen carefully and give them time to think things through
(From PersonalityType.com)