גַּרְתִּי: לֹא נַעֲשֵׂיתִי שַׂר וְחָשׁוּב אֶלָּא גֵּר, דָּבָר אַחֵר גַּרְתִּי בְּגִימַטְרִיָּא תרי"ג, כְּלוֹמַר, עִם לָבָן גַּרְתִּי וְתַרְיַ"ג מִצְוֹת שָׁמַרְתִּי וְלֹא לָמַדְתִּי מִמַּעֲשָׂיו הָרָעִים. Rashi has two interpretations if גרתי means a stranger of a reference to the תרי"ג mitzvot. The Chassidic masters (Likutay Sichos volume 3, Sfas Emes) say these are not two different interpretations but they go hand in hand. How did Yaakov manage to keep the mitzvot, the way of his forbearers? Because he viewed himself as a stranger in the house of Lavan, not belonging there and therefore he did not get affected by the Lavanik ways.
That is the idea of why Yaakov set up erev techumim to indicate that his head was not necessarily where his body was (as mentioned in the past.)