what tricks do the experienced hands in the boats play on the raw hands ?

They relax and tell the younger hands stories:

I notice that most of the old river hands are similarly retiring, whenever there is any stiff pulling to be done. You can always tell the old river hand by the way in which he stretches himself out upon the cushions at the bottom of the boat, and encourages the rowers by telling them anecdotes about the marvellous feats he performed last season.