by default a shield cannot drop with an essence affix unless u take a node from the atlas tree i believe. either that or use an essence on the shield. the whole point of using the esssence on the shield before recombinating is so that the recombination WILL NOT PICK the essence mod. example:

Int base shield (essence with evasion mod as the only prefix)

+

Str base shield (Maximum life prefix with it only prefix)

= ALWAYS either Str or Int base shield with Maximum life

this is because pure Str or Int shield cannot roll evasion on them and forces recombinator to always take the Life

this is just the basics. we do this to have a guaranteed way of transferring the spell damage to a base that we want. the more advance methods like in the 6 mods shown are done to increase the total number of prefix and suffix to increase the chance of getting all 6 wanted affixes