Actually the title is completely misleading because this has nothing to do with knitting. Here is the actual puzzle.
Get off one.
Off the top of one.
Keep one up.
Note I shall add some easier ones as time goes by unless someone gets it first.
(a) What sort of thing are these "ones" and "twos"? ---- ----- (4, 5)
(b) What common linguistic phenomenon are they part of? ------ (5)