Leadership Principles for the Contingent Engineering Workforce

Engineering managers who utilize contingent engineering workers do not have any formal, legal supervisor-employee hierarchical relationship with them. However, engineering managers often assume a leadership role in making work assignments, directing activities, evaluating work products, and obtaining the best performance from contingent workers. How can engineering managers best provide effective leadership to contingent engineering workers without incurring undesirable risk? What should engineering managers consider in selecting optimal leadership approaches and tactics for the contingent engineering workforce?