Because of the low amount of bad carbs and the high amount of fiber, sambar has a good Glycemic Index (GI) number. (A good GI number is a low one.) This is something that "The Healthy Indian Diet" book explains. But if you want to learn about the significance of the Glycemic Index for one's health now, you need to read Ms. Shyamal Leonard's article in India Currents magazine. (Read on.)
"Low-GI foods, by virtue of their slow digestion and absorption, produce gradual rises in blood sugar and insulin levels, and have proven benefits for health. With the majority of charts, a number lower than 55 means that the ranking is considered low, if the ranking is between 55 and 70 it is middle range, and above 70 is considered a high ranking. There may be some foods which cause a ranking above 100; this would normally interpret as a food that causes a higher spike in blood sugar levels."
Click here to read the article. You can learn more about the Glycemic Index in this American Journal of Clinical Nutrition article.
The Glycemic Load (GL) is a better indicator of how a particular food will affect your blood sugar than GI because GL incorporates the amount of that particular food you would eat. That said, the GI is an easy number to use when making decisions at the grocery store or kitchen counter.
A final thought. If you eat sambar with white rice, the benefit of a low GI food (which is good) is counteracted by a high GI food (which is bad, as white rice sold in industrialized countries has a GI of ~64, while pigeon peas -- the main ingredient of sambar -- has a GI of ~22). It's better for our health to eat sambar with brown rice, which has a lower GI of ~55.