What is a tip-up
A tip-up is a rig with free-running line and a mechanical bite signal. Everything in it comes down to one movement: the reel turns, and the flag pops up.
How it's built
The base either lies on the ice or a stand rises over the hole; on it sits a reel loaded with line and a spring-loaded arm carrying the flag. The arm is cocked and tucked under the reel: as long as it holds still, the arm stays pinned down.
How the signal works
The predator takes the baitfish and moves off, the line pulls the reel, the reel turns — and releases the arm. It springs up straight, the flag stands vertical, and it's visible from a distance. The signal is mechanical and needs neither a battery nor attention.
Why the reel has to turn freely
A stiff reel means resistance, and resistance means a dropped baitfish. So the spindle is kept clean and dry, and the line is wound in even coils: a loop on the reel stops the run at the worst possible moment, right after the fish has taken off.
What frost does to it
The hole freezes over and the line freezes to its edge; the reel gets covered in snow; water on the spindle turns to ice. That's exactly why the hole is dusted with snow — a hole covered with snow freezes over more slowly than a bare one. The rig is checked by hand after every bite.
Two designs
One sits flat on the ice and covers the hole: it freezes less, keeps the hole open underneath for longer, and in shallow water doesn't send a shaft of light down that scares fish off. The other sits on a stand over the hole: it's quicker to set and the flag can be seen from further off, but the wind tears it loose more often. The choice comes down to whichever is the bigger problem that day — frost or wind.
What the flag shows, and what it doesn't
The flag says the reel has turned, and nothing more — much like a flagless tip-up, except that one doesn't say even that much. A baitfish simply swimming around can turn the reel too, and a wary predator may not turn it at all. So a raised flag isn't rushed: the line is watched first, to see whether it keeps running out.
What to do with the slack line
The line that ran out on the bite lies on the ice and freezes solid within minutes. So after every tip-up that's fired, the wet line isn't wound straight back — it's drawn through the fingers first to let the water run off, or next time it will run out in jerks.