Read 系 (not xì) whenever it means 'to tie'. The jì reading appears in concrete fastening verbs: 系鞋带 (tie shoelaces), 系领带 (knot a necktie), 系安全带 (fasten a seatbelt), 系扣子 (button up). Anywhere the action is physical tying, read jì.
usage
The jì reading corresponds to traditional 繫, which the 1956 simplification merged into 系. Taiwan and Hong Kong texts still write 繫鞋帶 with the full form. Mainland readers see only 系 and rely on context to pick the reading.
The silk-skein radical, indexing component for 系. It pictures twisted silk threads with a knot at the top. Combined with the hand stroke above, the whole graph reads 'a hand tying the thread' — origin of the senses 'connect, link, system, tie.'
A leftward downstroke at the very top representing a hand reaching down to grasp the silk below. This abbreviated hand-graph is what distinguishes 系 from plain 糸: it shows someone tying or attaching the thread, not just the thread itself.
