The Gemara in Masechet Berachot establishes a Halacha exempting workers from certain sections of Birkat Ha'mazon. Employers in ancient times were very strict regarding the time taken by their employees to eat, and therefore the Sages ordained that workers recite an abridged Birkat Ha'mazon rather than take the time to recite the complete text. (This Halacha does not apply today, when employers are more flexible and allow their workers sufficient time to break for lunch and recite the complete Birkat Ha'mazon.)
Why did the Rabbis not simply ordain that the workers recite Birkat Ha'mazon as they work? Assuming a worker knows the text by heart, this appears to be a preferable solution to skipping certain sections of Birkat Ha'mazon.
The answer is that when one recites a Beracha, he must do so with full concentration and should not occupy himself with anything else. People commonly recite "Asher Yatzar" upon leaving the restroom as they're rushing back to whatever they were doing. This is incorrect. A person should stop and concentrate on the Beracha, without performing any other tasks or signal or make hand motions to other people. The Sages therefore preferred that workers recite an abridged Birkat Ha'mazon with proper concentration, rather than allow them to recite the complete text while they perform their assigned tasks.
Rabbi Dan Segal told that when the Chazon Ish (Rabbi Avraham Yeshaya Karelitz, Lithuania-Israel, 1879-1954) took ill, he felt that reciting Berachot with his usual intensity could endanger his health, and he therefore ate without reciting Berachot. So important it is to recite Berachot with full concentration that the Chazon Ish saw it appropriate to omit Berachot altogether rather than recite them with anything less than his usual level of intensity.
This Halacha applies as well to people listening to Berachot. Very often, as people listen to Kiddush at the Shabbat table they are involved in other activities, such as motioning to the children or arranging the food or utensils on the table. This is improper; everybody listening to Kiddush must stop whatever they are doing and give their full attention to the recitation of the Beracha. Similarly, one should not dry his hands while making the Beracha of Al Netilat Yadayim.
Summary: Whenever one recites or listens to a Beracha, he must ensure not to be involved in any other activity, including hand motions and the like; he should instead concentrate intently on the recitation of the Beracha.
