Substring selector box
The substring selector box is a combo box. In the textfield of the combo box you will write substrings. Only options that match all the substrings will be shown in the dropdown list. The dropdown list will dynamically update every time you enter/change a letter.
Searching in an address book. Your address book has a lot of persons called Peter and at lot of persons called Smith, but you only have very few called Peter Smith. In the combo box you can write 'pet smi' or maybe 'er mit' if that is faster. The substrings will not need to start nor end on word boundary.
Searching redial list. If you call a lot of persons that are not in your contact list, you may remember part of the number that is reasonable unique. Using the Substring selector box you enter the part you remember and a list of the matching numbers is shown.