Nowadays, people are not used to reclining on couches while eating, and it is therefore necessary to explain how to perform hasava in a chair on the Seder night. Instead of sitting erect with one’s back against the seat back, one slides his bottom forward to the middle of the seat so that he may lean back against the chair back, and tilts to the left. Le-khatḥila, it is best to recline on an upholstered chair with armrests or to use a pillow to make sitting more comfortable. Regardless, anyone who uses a chair with a backrest fulfills his obligation by reclining against the back of the chair and tilting to the left, for this is an expression of freedom. After all, a receptionist, for example, must sit erect in his chair in order to be ready to do his work, but one who has no burdens can stretch out, lean back, and rest freely.
The reason for reclining to the left is that it is easier to eat this way; with the left hand and back reclining against the chair, the right hand, which we generally use, remains free to hold the matza or wine. Additionally, some say that one who reclines to the right runs the risk of choking on his food and suffocating. Because of this risk, it was ruled that even a left-handed person must recline to the left and use his right hand on the Seder night. Be-di’avad, a right-handed person who mistakenly reclines to his right does not fulfill his obligation, but a left-handed person who reclines to the right does (SA 472:3, MB ad loc. 10-11).
If one is sitting in the company of his rabbi or a leading Torah sage, he must ask his permission before reclining, because hasava contains an expression of disrespect and irreverence toward the rabbi, and the mitzva to honor the Torah takes precedence over the mitzva of hasava. But if one receives permission from his rabbi, then hasava no longer constitutes a display of disrespect (SA 472: 5).