How having sickle cell trait would provide resistance to malarial parasites?
I would say that there are at least 2 explanations:  


Higher levels of free heme in the blood of individuals with sickle cell anaemia/sickle cell anaemia trait. Free heme is toxic to Plasmodium.sp Science. 1981 Nov 6;214(4521):667-9., detailed mechanism is described here: J Mol Med (Berl). 2008 Oct;86(10):1097-111. This effect is also observed in people with other diseases affecting stability of erythrocyte's membrane, such as thalasemia or spherocytosis. 
It is also possible that, when erythrocyte's membrane is prone to disruption, Plasmodium.sp  antigenes are present in higher concentrations and may be faster presented to the immune cells.