You grapple the relay with your arm (which if you've made it to a relay you know how to do) and then press the light punch key (default F on keyboard/mouse setup).
However - surely the tutorial is still given to you near the beginning of the game - which tells you exactly how to "hack" the relays... it's not a secret or anything - there's no real "hacking" or anything even remotely tricky going on - it's just like moving or punching or whatever, just hit the button and it's done, no?
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