What does "legal gambit" mean as a metaphor?

OED:

Etymology: Probably partly < Spanish gambito gambit in chess (1561 with reference to chess, in Ruy López Libro..del juego del Axedres iii. vii f. 104),

and partly < its etymon Italian gambetto a tripping up in wrestling (14th cent.), gambit in chess (a1575, earliest in form gambitto , probably after Spanish) < gamba leg (see jamb n.) + -etto (see -et suffix1).

A gambit was thus a stratagem in chess. But became a metaphor for any trick or plan:

Gambit (n.)

2. (In early use in extended metaphors of sense 1; cf. opening gambit n. [...])

(*2a. A remark intended to initiate or change the direction of a conversation or discussion. Frequently in opening gambit - n. 1911 P. Gibbon Margaret Harding 44 She had caught from Mrs. Jakes the first rule of polite conversation... ‘Sun burning plenty; how's Missis?’ was her usual opening gambit.)

But, in your example, it is in the following sense that the metaphorical gambit is used

2 b. A plan, stratagem, or ploy that is calculated to gain an advantage, esp. at the outset of a contest, negotiation, etc.

1925 C. Burt Young Delinquent viii. 338 His usual gambit was to set his fellow office boys bidding for the smaller trinkets, and then with a show of generosity to return the purchase money.