Pak vs India champions trophy 2018

So this is last edition of hockey champions trophy .

PK lost to india 4 - 0. Upto 3Qs it was 1 -0 .
In last Q our coach interchanged goalkeeper with extra forward ( means no goal keeper ) . and we conceeded 3 more goals in 6 minutes.

I cannot understand that in existing hoceky rules ( No offside ) how sumone can make such decision.