This, some argue, gave employers the ability to require more from Bracero workers based on a threat or promise they would be sent back to Mexico.
Employers threatened deportation to Mexico in order to drive wages down further and institute extremely harsh requirements. 
mnli hypothesis: Employers threatened deportation to Mexico in order to drive wages down further and institute extremely harsh requirements.  premise: This, some argue, gave employers the ability to require more from Bracero workers based on a threat or promise they would be sent back to Mexico.