Combat helmets are protective head gears used to protect soldiers from head injuries on the battlefield. Currently, these helmets are only designed for ballistic and blunt impact. The increasing numbers of blast-related head injuries in asymmetric warfare activities and modern battlefield operations show the necessity for designing these helmets for blast loading. When a blast wave interacts with a helmet-protected head, several interactions such as wave focusing in concave facial structures, flow reunion, and underwash effect causes pressure amplification around the head. These pressure amplification may increase the chances of injuries due to blast waves. The present study mainly focuses on understanding the underwash effect through experimental and numerical investigations using idealized helmet-head models. Underwash effect is the pressure amplification happening in the gap between head and helmet. The organization of the study is as follows. The first part deals with experimentally quantifying amplification factors due to the underwash effect for various head-helmet orientations. The Bottom orientation was found to be the most critical orientation in terms of pressure amplification due to the underwash effect. The second part deals with visualization of the underwash effect using schlieren imaging to understand how pressure amplification occurs in the gap. Interestingly certain flow features such as back and forth reflections in the gap, which are not mentioned in the earlier hypothesis, are observed. The final part deals with numerically investigating the effect of different gap geometries on the underwash effect. The gap size at the crown does not influence pressure amplification, but the gap size at the edges has a considerable influence. Also, a correlation between localized amplification in surface pressure field and hydrostatic stress field in the brain is obtained using coupled Eulerian-Lagrangian analysis in ABAQUS/explicit. The observations from this study will help in the design of protective gear to mitigate the adverse effects of combat helmets under blast loading.