Most popular cap for those aRM programs now are 5/2/5. That means if could go up 5% 5yrs(or 7yrs) later. And every year after 6th(or 8th) year, the CAP will be 2%. And the life CAP is 5%.
Let's say your 5/1 ARM is 2.5%. With margin 2.25% and the index is 1 year LIBOR. At the ending 5th year, the 1 yr LIBOR index is 4%. Then your rate will be 6.25%. But if 1 yr index is 10%, then your rate will be 7.5%.
Then if 6th yr rate is 6.25%, and at the end of 6th yr, if libor index is 7%. Your rate will be 6.25% + 2% = 8.25%.
The lender will always use min(index+margin, current rate + CAP). CAP is 5/2/5.