be cautious, careful, alert
惕 centers on a state of watchful readiness: being cautious, careful, and alert to avoid danger or mistakes.
惕 combines 忄 (heart/mind) with 易, which likely contributes a phonetic element. The heart radical suggests a mental state of caution or alertness.
The heart radical 忄 beside 易 suggests a mind that is easily startled into alertness. Picture a cautious heart that stays on guard.
For テキ, imagine a careful technician (テキ) inspecting equipment with a watchful eye: tech -> テキ, and the technician's alertness prevents mistakes.