An experimental work and a physical analysis dedicated to the study of a low density jet subjected to a time varying crossflow with high acceleration/deceleration levels are presented in this paper. Relevant nondimensional numbers are derived and show that unsteady effects associated with the presence of the jet in the acceleration field have noticeable consequences on the flapping of the jet. The Schlieren technique is applied in the test section of a square duct to obtain time resolved images of the jet. Analysis of the results is focused on the influence of the unsteady effects on the global dynamic behaviour of the jet in the near field. The interaction between the jet and the crossflow is analysed in three contrasted situations corresponding to different values of the jet outlet velocity U0 . We predict and observe an increase of the jet deflection during the acceleration phase and a competition between drag and acceleration during the deceleration. This competition is particularly clear for the two lowest ejection velocities of the jet and we have shown that the jet is initially deflected upstream the nozzle. The influence of exit jet injection angle is finally considered. We show that upstream or downstream injections induce a very strong modification of the mixing process of the jet fluid with the pulsed crossflow.