I can't be the only one who had to look up this evocative phrase. The dictionary wasn't terribly helpful:
frog-march vt [Informal, Chiefly Brit.] to grasp by the arms and force to walk along
So why "frog"? Word-wizard comes through:
Cassell’s Dictionary of Slang
FROGMARCH verb [mid-19th century and still in use] to carry someone face down, one person holding onto each limb; used on drunks or recalcitrant prisoners.
Random House Historical Dictionary of American SlangFROG-MARCH verb [shift and alteration of “frog’s march” (not recorded in U.S.), as in 1871, 1873 quotations] Especially ‘police.’ to carry (a resisting person) face downward by the arms and legs; (hence now solely) to propel (a resisting person) forward, as by seizing his collar and the seat of his trousers or by pinioning his arms behind his back. [1871 in OED: “They did not give the defendant the frog’s march.”] 1873 ‘Slang Dictionary’ by Hotten: “Frog’s March,” the manner in which four or more policemen carry a drunken or turbulent man to the station-house. The victim is held face downwards, one constable being at each shoulder, while the others hold on above the knees. Often…another…officer… beats time…on the recalcitrant hero’s posteriors.] 1969 in OEDS: “He. . . took me by the collar and the seat of my pants and frogmarched me the length of the café.” 1992 Newsday (CNN-TV) (Dec. 9): “Tightly bound and frogmarched away.”