תניא סומכוס אומר כל המאריך באחד מאריכין לו ימיו ושנותיו אמר רב אחא בר יעקב ובדלי"ת אמר רב אשי ובלבד שלא יחטוף בחי"ת ר' ירמיה הוה יתיב קמיה דר' [חייא בר אבא] חזייה דהוה מאריך טובא א"ל כיון דאמליכתיה למעלה ולמטה ולארבע רוחות השמים תו לא צריכת:
Or, in English:
It has been taught: Symmachus says: Whoever prolongs the word ehad [one]. has his days and years prolonged. R. Aha b. Jacob said: [He must dwell] on the daleth.12 R. Ashi said: Provided he does not slur over the heth.13 R. Jeremiah was once sitting before R. Hiyya b. Abba, and the latter saw that he was prolonging [the word ehad] very much. He said to him: Once you have declared Him king14 over [all that is] above and below and over the four quarters of the 'heaven, no more is required.
Perhaps daled then signifies the daled ruchos haolam. A word on how one can extend the daled. In Mishnaic and Talmudic times, there was a pronounced distinction between daled with a dagesh kal (a dot inside) and without a dagesh kal, parallel to tav vs. sav or bet vs. vet. With a dagesh, it is a plosive, and is pronounced just as we pronounce a daled. Without a dagesh, it is a fricative, and is pronounced like the 'th' in the word 'either'.
It is thus possible to extend the daled. Nowadays, in order to fulfill this Talmudic instruction, we could pronounce our daleds in Shema (or in that first pasuk) as fricatives. Otherwise, we can simply assert that it is no longer possible to fulfill this Talmudic instruction, and not attempt it.
I've seen people, who do not know about the fricative dh, who try to extend the plosive daled. This is not really possible, and they and up just saying a harsh DI sound at the end. Look in the Magen Avraham about an instruction lehadgish the daled, which he clarifies does not mean to make it into a dagesh chazak (strong dagesh, which geminates, or doubles, the pronunciation of the letter), but to distinguish it clearly from a resh.
I've seen some people extend the kamatz before the daled. That also is not entirely optimal, since Rashi writes:
ובדל"ת - ולא בחי"ת דכל כמה דאמר אח בלא דל"ת לא משתמע מידי
And thus, before you actually reach the daled, you are not really extending the echad.
It is unclear what is meant by:
ובלבד שלא יחטוף בחי"ת
in the gemara. Does this mean the letter, or does this mean the vowel under the letter? Rashi writes:
ובלבד שלא יחטוף בחי"ת - בשביל אריכות הדל"ת לא ימהר בקריאתה שלא יקראנה בחטף בלא פתח ואין זה כלום:
In other words, if you change the kamatz under the chet into a sheva -- by which I think he might even mean a sheva nach. (Sometimes, a patach was written next to the sheva to show it was pronounced, na, rather than being specifically a chataf patach.)
In Shulchan Aruch, they also speak of extended the letter chet itself somewhat, and there is a dispute whether this is something one should do. I wonder if this machlokes somehow is related to different pronunciations of chet in different geographical regions. In places where chet has merged with the fricative chaf, it is possible to extend it. (And perhaps one can even them reinterpret the gemara's ובלבד שלא יחטוף בחי"ת as such an instruction.) Meanwhile, in places where chet retained its status as a guttural, which is hard to pronounce and extend, they would not have reinterpreted the gemara in this manner.