#K, 10 bytes

    ~/1_'-':'=

Note this is a function, so it needs to be *called*:

      ~/1_'-':'="aaaaaabbbbbb"
    1
      ~/1_'-':'="aba"
    0

`=` groups its arguments, so `="aaaaaabbbbbb"` produces `"ab"!(0 1 2 3 4 5;6 7 8 9 10 11)` and `="aba"` returns `"ab"!(0 2;,1)`

`-':'` is *minus eachprior each*. `-':` is a good way to find out if a series is increasing (or decreasing). `-':'="aaaaaabbbbbb"` gives us `"ab"!(0 1 1 1 1 1;6 1 1 1 1 1)` and `-':'="aba"` gives us `"ab"!(0 2;,1)`

`1_'` is *one drop each* which pops the first element off each list.

`~/` is *match over*.