1)
What is the significance of tying them on your fingers?
1.
Malbim: Everything that you do with your fingers should be according to Torah.
2)
What is the significance of writing them on the tablet of your heart?
1.
Malbim: All thoughts of your heart should be according to the ways of Chachmah. The heart is the power in the Nefesh that rules. The images of Chachmah must be written on it like "Elokim's writing carved on the Luchos" (Shemos 32:16), to cease 1 the images of desire that come on the heart. I.e. both thoughts, and actions of the fingers, must be according to the Torah's counsel.
1
Surely the text should say 'Lehashbis', and not Lehashbi'ach. (PF)
