  We show that the termination of the star formation process by winds from
massive stars in protocluster forming clumps imposes dual constraints on the
star formation efficiencies (SFEs) and stellar age spreads ($\Delta \tau_{*}$)
in stellar clusters. We have considered two main classes of clump models. One
class of models in one in which the core formation efficiency (CFE) per unit
time and as a consequence the star formation rate (SFR) is constant in time and
another class of models in which the CFE per unit time, and as a consequence
the SFR, increases with time. Models with an increasing mode of star formation
yield shorter age spreads (a few 0.1 Myrs) and typically higher SFEs than
models in which star formation is uniform in time. We find that the former
models reproduce remarkably well the SFE$-\Delta \tau_{*}$ values of starburst
clusters such as NGC 3603 YC and Westerlund 1, while the latter describe better
the star formation process in lower density environments such as in the Orion
Nebula Cluster. We also show that the SFE and $\Delta \tau_{*}$ of massive
clusters are expected to be higher in low metallicity environments. This could
be tested with future large extragalactic surveys of stellar clusters. We
advocate that placing a stellar cluster on the SFE-$\Delta \tau_{*}$ diagram is
a powerful method to distinguish between different stellar clusters formation
scenarios such as between generic gravitational instability of a gas
cloud/clump or as the result of cloud-cloud collisions. It is also a very
useful tool for testing star formation theories and numerical models versus the
observations.
