The Arizal (Rav Yishak Luria of Safed, 1534-1572) taught that when one recites the first verse of "Shema," and he reaches the final letter of this verse – the "Dalet" at the end of "Ehad" – he should have in mind that he accepts upon himself the four "Mitot Bet Din" – forms of capital punishment. The Gematria of "Dalet" is 4, and so when one recites this letter in "Ehad," it is proper to have in mind that he accepts, if he is deserving of such, the four different forms of execution that used to be administered by courts – "Sekila" (stoning), "Serefa" (burning), "Hereg" (decapitation) and "Henek" (strangulation). This Kavana (intention) is written in a number of editions of the Siddur.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in his work Od Yosef Hai, references his discussion in a different work – Torah Li’shmah – as to whether one must have this entire Kavana while reciting the "Dalet." It might be difficult to pronounce the letter "Dalet" long enough to have in mind all four "Mitot Bet Din" during the recitation of this letter. Does it suffice to begin having this Kavana while reciting the "Dalet" and then completing it afterward, or must the entire Kavana be completed while pronouncing the "Dalet"?
The Ben Ish Hai cites a passage from the Zohar implying that one should have this entire Kavana in mind while pronouncing the "Dalet," and not afterward. Nevertheless, the Ben Ish Hai writes, if one could not hold the "Dalet" long enough to complete this Kavana, he may complete the Kavana afterward, without saying anything.
It must be emphasized that this discussion of the Ben Ish Hai appears in his work Od Yosef Hai, which was written not for great scholars of Kabbalah, but rather for ordinary laymen in Baghdad. This Kavana is something which everybody is capable of – especially in light of the Ben Ish Hai’s lenient ruling that one can complete it, if necessary, after he finished pronouncing the "Dalet." Therefore, given the great benefits of this practice, it is worthwhile for one to make an endeavor to have this special Kavana every time he recites Shema.
Summary: Kabbalistic tradition teaches that when one pronounces the letter "Dalet" in "Ehad" at the end of the first verse of the Shema, he should have in mind that he accepts upon himself the four forms of capital punishment ("Sekila," "Serefa," "Hereg" and "Henek"). One should try to think of all four while pronouncing the letter "Dalet," but if one cannot hold the letter "Dalet" long enough, he may complete this intention after he finished pronouncing the "Dalet." This practice yields great benefits, and so everyone should endeavor to have this intention each time he recites Shema.