Lassi and chaas start from the same cultured yogurt, yet one pours like a milkshake and the other like water. The difference is not a different recipe but a different amount of physical disruption.
Yogurt is a gel, not a liquid
Set yogurt is milk protein locked into a continuous network that traps water inside it. That network is why an undisturbed cup of yogurt holds a spoon upright rather than flowing.
Whisking breaks the network into fragments. The fragments still hold water internally, so a lightly beaten yogurt stays thick even after some liquid is added to loosen it.
Beat harder and longer and the fragments shear down further, releasing the water they held. The drink thins out sharply at that point, which is the state chaas is made in.
Water is added at two different scales
Sweet lassi typically takes a small fraction of water relative to the yogurt, enough to make it drinkable but not enough to disperse the protein fragments. It stays viscous and coats the glass.
Chaas is closer to the reverse ratio, with water dominating. The protein fragments end up suspended in a large volume, and the drink behaves as a thin seasoned liquid rather than a thick one.
American yogurt changes the starting point
Most US supermarket yogurt is strained to some degree, and Greek-style tubs are strained heavily. Strained yogurt has already lost whey, so it makes a much thicker lassi at the same water ratio.
Plain whole-milk yogurt without added thickeners is the closer match to Indian dahi. Products carrying gelatin, pectin or modified starch resist thinning and can turn slimy when beaten hard.
If only strained yogurt is available, adding water in stages and tasting for body works better than following a fixed ratio written for unstrained dahi.
Salt and sugar do more than flavor
Salt in chaas increases the ionic strength of the liquid, which encourages protein fragments to draw together and settle. That is why a salted drink separates faster if it stands.
Sugar in lassi raises viscosity slightly on its own and slows separation. It also blunts the sourness of a tart culture, which is why sweet versions tolerate older yogurt better.
Separation is not spoilage
A layer of clear liquid on top of a standing glass is whey escaping the fragmented network, a physical change rather than a sign that the drink has gone off.
Stirring or a brief re-whisk redistributes it. Because both drinks are dairy that sits at room temperature while being made, general refrigerated-dairy handling guidance from food safety authorities is the sensible reference for how long to keep a batch.