The obligation of "Heseba" at the Seder requires one to lean while drinking each of the four cups of wine, and while eating the Masa, Korech and Afikoman. Even if one finds it difficult to lean, he should make the effort to fulfill this obligation.
What is the proper way to lean for Heseba?
In ancient times, people would perform Heseba by nearly lying down. They would eat on the floor around low tables, and they would lean on one another or on the tables in front of them. According to the strict Halacha, however, it suffices to lean at a 45? angle. Heseba also requires leaning on something, as opposed to simply tilting one’s body and leaving it suspended in the air. One should lean to the left side and rest against either the back of the chair, a pillow or another piece of furniture, such as the table. Some people simply tilt their head to the side or move their body slightly; this is incorrect. One must lean at least to the position of a 45? angle and rest on something in order to fulfill the obligation of Heseba.
(Based on the rulings of Hacham Ben Sion Abba Shaul in his Or Le’Sion, vol. 3, Siman 15; listen to audio recording for precise citation)
Summary: To fulfill the obligation of Heseba (leaning) at the Seder, one must lean to the left side, at least at a 45? angle, and rest on something, like the back of the chair, a pillow or the table.
