It is also related that the squeeze in the grave is the atonement for the sins that a believer has committed.
Shaikh Sadooq quotes Imam Ja’far-as-Sadiq (a.s.) as saying, that a Scholar (Aalim) was told in the grave that he would be lashed a hundred times as punishment. The Scholar replied that he had no strength to bear that much. The Angels started reducing the number till only one lash remained. The Scholar asked the reason for this punishment.
He was told that once he had recited Namaz without performing Wuzu’, and once he passed by an old and feeble man who requested him to give him a helping hand, but he paid no heed. The Scholar consented to be lashed. He was lashed only once which filled his entire grave with fire.
The Holy Prophet (s.a.w.s.) says that, if a believer (Mo’min) does not help his believer brother even after having capacity to do so, Allah will send a large serpent named ‘Shaja’ in his grave, who will keep biting at his fingers. In another narration it is stated that the serpent will keep on biting his fingers even though his sin will have been forgiven.