We report on theoretical and experimental study of pulsed lasing driven by synchronous pumping of an Yb-fiber laser. We show that even simple sine-wave modulation of the pump power with moderate depth (~50%) can result in shaping of a regular train of discrete laser pulses. Moreover, generated pulses are typically much shorter than pumping pulses. The pulse shortening factor rises with the modulation depth of pumping. It reached 5 in our setup. Thus, ~200-ns laser pulses were generated with sub-MHz sine-wave modulation of the pump power. Theoretical modelling predicts possibility of further pulse shortening (up to a factor of 11) by increasing maximal pump power and modulation index. Such shortening can be attributed to self-induced gain discrimination of the temporal pulse profile.