Throughout the prayers, one must be particular about reading the words and sentences correctly. This is particularly relevant when reading the Shema'. Ribbi Hama Ben Hanina says in the Gemara of Berakhoth (15b), something astonishing, that whoever reads the Shema' and is particular about its letters, (to read them correctly), will have Gehinnam (hell) cooled for him.
When reading the Shema', joining the last letter of some words with the first letter of the following word, can change the meaning. Pausing in the wrong place within a sentence or joining words that require a pause between them, can also change the meaning.
One must, therefore, read the Shema' slowly and deliberately and fight the temptation to rush through it. The reason why our Rabbis of blessed memory tell us that Gehinnam is cooled for one who is particular about the reading of the Shema' is precisely because the nature of a person is to rush through the reading. When he reverses his nature and reads it in a calm and precise manner, ensuring that all the letters are said correctly, measure for measure, hell reverses its nature and cools down for him.
(See Shulhan 'Arukh Orah Hayyim 61:5, 22. Kaf Hahayyim, ibid., 56)
gehinom, gehinam, gehinnam, shema’ Yisrael, Shma Yisroel, being careful to read correctly