The Sages ordained that one recline while eating matza and drinking wine at the Seder, because in every generation one must give the appearance of having just been freed from Egyptian bondage, as it is stated: “He rescued us from there” (Devarim 6:23). This reclining is called “hasava,” and it was instituted so that a sense of liberation would be apparent in one’s behavior (MT, Laws of Ĥametz and Matza 7:6-7).
One who is burdened with a task usually sits upright so that he will be able to rise at once when the time comes for him to carry out his work. Even though sitting upright requires some effort on the part of the back muscles and creates a state of constant tension, the need to be ready for action requires this state of alertness. However, one who has no burdens can lean back and lie on his side restfully, allowing all of his back muscles to relax. This is how we eat on the Seder night, in the manner of the liberated.
In the past, people would sit on pillows and cushions, which meant that sitting erect indeed required effort. In such circumstances, hasava – the position between sitting and lying down, where the entire body reclines on a couch or on pillows and cushions – was very comfortable and demonstrated a sense of freedom. But today, people sit on chairs and are not accustomed to reclining on couches or eating while reclining to the side. In fact, if one were to eat while reclining on a couch these days, it would be more burdensome than comfortable. Therefore, according to Raavya and Raavan, two leading Rishonim, there is no mitzva to practice hasava nowadays. However, most Rishonim maintain that since the Sages mandated reclining at the Seder, their ordinance stands firm, and there continues to be a mitzva to eat matza and drink the four cups of wine while reclining (Rambam, Rosh, Tur, SA 472:2). Today we recline by leaning against the back left of our chairs.
The requirement of hasava applies when eating a kezayit of matza, a kezayit of korekh, a kezayit of afikoman, and while drinking the four cups of wine. It is commendable to recline during the rest of the meal as well, but if one finds this uncomfortable, there is no obligation (MT, Laws of Ĥametz and Matza 7:8). One need not recline while eating the maror (MB 475:14, based on Beit Yosef). One should not recline while reciting Birkat Ha-mazon, which must be recited with awe and reverence (SA 183:9). Likewise, it is customary to refrain from hasava while reading the Hagada, so that it is recited with full concentration and seriousness (MB 473:81, based on Shlah).