In this research piece, one of the trades on Page 31 is Pay EUR 5y2y-7y3y-10y5y. What is the meaning of this notation? I guess it is a fly trade on three forward rates, but it is confusing that the three rates have different starting dates (2y vs 3y vs 5y). What is the intuition behind choosing such flies?
You can have a look at the fly in nordea.
It's the same logic, just different tenors. If you google paying the belly you also find info. Essentially, by selling the short- and long-term (the wings) of the yield curve and buying the medium term (the belly) at the same time, you gain if the medium-term rates increase relatively faster than rates on the other two.
Your research paper also has a similar strategy (10y5y-15y10y-25y5y) which is explained in more detail (exhibit 27).