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I had an impression that they might be talking what a digital I/O pin looks on oscilloscope.

Scopes lock at first rising edge after last horizontal scan, so display starts at H, then drops to L after how long CPU held that pin high. That creates ￣￣l＿ lines on the screen superimposed to one another, X position “wiggling” ￣lll＿ depending on how many consecutive H bits just happened to be sent.

When the CPU halts, the pin would flatline at H or L and you’ll know.