下跪 (xiàguì) is a more formal way to say 'to kneel down.' 跪下 (guì xià) is more colloquial. In internet slang, 跪了 or 跪求 expresses extreme admiration or desperate pleading.
memory
足 (foot) radical + 危 (danger) — your feet are in a dangerous position, folded under you as you kneel.
