Dr Ian Plummer

http://www.oxfordcroquet.com/laws/6th/9.asp
Oxford Croquet

9. ELECTION OF STRIKER'S BALL

1. RIGHT TO PLAY EITHER BALL After all four balls have been played into the game under Law 8(b), the striker may elect at the start of any turn to play that turn with either of his balls (but see Law 37(a) for handicap play).
2. HOW ELECTION IS MADE The election is made by:
1. lifting a ball that is not in contact with another ball, in accordance with Law 13 (wiring lift) (or Law 36 (optional lift or contact in advanced play)); or
2. playing a stroke.
In each case the ball so elected becomes the striker's ball for that turn and the striker must not then strike the partner ball. If he does so, Law 26 applies.
3. LIFTING A BALL A player lifts a ball by deliberately moving it from its position in a manner other than that used for playing a stroke.