3D Touch capable models (6s to x) had actual pressure sensors embedded in the screen assembly, which meant there was another input channel (how hard you pressed) that apple mapped to an extra commands, like a right click. To highlight text with it, you could press slightly harder anywhere on the keyboard and that would start cursor movement mode (like holding on the spacebar on newer models), then, with the cursor on the word you wanted to start highlighting from, you pressed a bit harder and it would highlight from that word on until you released the keyboard.
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