Byasana is a type of fasting where one eats two meals a day (twice a day, but with restraint). During Paryushan and similar austerities, many Jains follow Byasana along with other rules to keep the fast pure and simple. The basic idea is to eat light, sattvic food and finish the last meal before sunset, while avoiding foods that harm the principle of ahimsa (non-violence) in ingredients and processing. ( jainknowledge.com)

What kinds of foods are typically allowed (general guidance):

• Plain, simple grain-based dishes: chapatis (made without oil/ghee), plain rice or khichdi prepared with minimal spices.
• Pulses and cereals: dals and pulses cooked plainly; flours from grains/pulses may be used in basic preparations.
• Boiled vegetables or vegetables without root parts, cooked without strong spices.
• Fresh fruits and simple salads (if allowed by the specific observance).
• Limited spices: salt and a few basic seasonings; avoid strong or heated spices, and avoid honey, alcohol, onion, garlic.
• Water: boiled water is often preferred; if drinking, do so within the daylight hours for those observing Byasana.

Important notes:

• No root vegetables (like onion, garlic, potatoes) are typically allowed during strict observances.
• Fermented foods and processed items are generally avoided.
• Dairy, oils, ghee, and rich dairy desserts are usually restricted or avoided in stricter practices, but exact allowances can vary by community and the specific fast you are observing.
• Always finish eating before sunset.

Sect differences (Digambar vs. Shvetambar):

• There are variations in what is permitted, especially regarding dairy, certain grains, or specific preparations. Some traditions keep dairy out more strictly, while others allow limited dairy in lighter fasts. The core aim remains non-violence, simplicity, and self-control.

If you want, I can point you to a JainKnowledge page that outlines the rules during Paryushan fasting and where Byasana is described. You can read more here