Toss a tennis racket in the air so it spins about its handle, and it flies straight. Spin it flat, like a frisbee, and it also flies straight. Now spin it about the third axis, the one that runs across the face from side to side. It flips. Every time. It does this in a gym, and it does it in orbit, where there is no gravity and no air to blame.
This block is spinning in empty space. Nothing pushes on it. Pick which axis it spins about, and use the slider for how far off-axis the first nudge is. The lime face is one big side, the grey face is the other, so you can see a flip.
Spin about the long axis or the short axis and the wobble stays tiny, however you nudge it. Spin about the middle axis and the same tiny nudge grows until the block turns over, then turns back, then over again, forever.
Any solid object has three special perpendicular axes, called principal axes. Each has a moment of inertia, a number saying how hard the object is to spin about that axis. For a block of length 3, width 2 and thickness 1, the long axis is easiest to spin, the thin axis (through the faces) is hardest, and the width axis sits in between. A phone, a book and a racket all have this same ordering.
The rule, known as the intermediate axis theorem or the tennis racket theorem, says that spinning about the smallest or the largest moment is stable, and spinning about the middle one is not. A free body keeps its angular momentum constant. The spin axis can still wander inside the body, and for the middle axis the wandering gets exponentially larger.
Euler's equations describe a free spinning body. For spin rates ω1, ω2, ω3 about the three axes, the rate of change of each spin depends on the product of the other two. Start with nearly all the spin about the middle axis, with tiny leftovers on the other two. The leftover on axis 1 grows the leftover on axis 3, and the leftover on axis 3 grows the leftover on axis 1. They feed each other. For the long and short axes the same feedback has the opposite sign, so it makes the leftovers swing back and forth instead of grow.
That is why the slider matters so little. A bigger nudge makes the first flip come sooner. A smaller one just delays it. Only a perfect start with exactly zero nudge would stay put, and nothing is perfect.
In 1985 the cosmonaut Vladimir Dzhanibekov, aboard the Salyut 7 space station, noticed that a wing nut spun off its bolt kept spinning smoothly, then suddenly flipped end over end, again and again. It became known as the Dzhanibekov effect. It is the same physics as the racket, seen in slow motion because there is no gravity to hurry the toss along.
Put a rubber band around a book to hold it shut, or use a closed box. Toss it gently over a bed so it spins about each of its three axes, one at a time. Two of the throws look clean. The third wobbles and flips, even if you try to throw it perfectly.
Next time you see a phone slip out of someone's hand and flip as it falls, you know the reason. It is not clumsiness. It is a theorem.