Short gantry 
I don`t konw if every konw gantry , it is huge crane for lifting 
it have a beam and two legs  with a span 

and our short is very similar this machinery,  is it? yes let me compare  it 

as the follow drawing show , it is a short gantry ,  have a beam with a interesting angle relative to a horizontal plane, this mean the yield supply by shorter . short leg 
mean the income of short ,and cover leg is the cost when cover . and the span it how long term you short ,and both legs is base on price line . if the cover leg longer than short leg , .it mean lost money using Short Gantry ,earn money oppositely .
now let us assemble this gantry on BTS price line ,as picture show , if the span of Short Gantry less than cycle of price of bts up and down. using short gantry have to bear risk from rapid fluctuation of bts price .
it is not encourage user with long-term value .
but if we use larger span Short gantry ,it is  larger opportunity earn money for user with long-term value .