Free rein

I needed no whip, or spur, for I was as eager as my rider: he saw it, and giving me a free rein, and leaning a little forward, we dashed after them.

[I]n the rise during the leap, just previous to the spring, no efforts whatever must be made by the rider to support the horse, or to lift him, but instead, she should simply hold the reins so lightly that his mouth can just be felt, which is called "giving a free rein."

So ceased the sea's uproar, when its grave Sire / Looked o'er th' expanse, and, riding on in light, / Flung free rein to his winged obedient car.

to give [a] free rein to

[H]e vvas pleas'd to lend / Free reines to mirth, and to ſuſpend thoſe cares / That claime ſuch intereſt in th' Imperiall brovv.

[Geoffrey] Chaucer gave a free rein to his poetical mirth.

If you have any influence, therefore, on members of Congress, on editors, on the creators of public opinion, on your neighbors, on the rank and file of your army, teach them that with Massachusetts bayonets, it is better to be insubordinate, and shoot a Colonel, than it is, unasked, unauthorized, and Heaven-damned, to turn themselves into hunters of slaves. (Loud and prolonged cheers.) Help the Government to dare to give free rein to the ardor of the people.

At the suggestion of the latter [Bardas], Michael [III] sought the assistance of Ignatius [of Constantinople] in an effort to force Theodora to enter a convent, in the hope of securing for himself an undivided authority and a free rein for his profligacy. The patriarch indignantly refused to be a party to such an outrage. Theodora, however, realizing the determination of her son to possess at any cost an undivided rule, voluntarily abdicated.

Such hedging is necessitated by the lack of in-depth knowledge of the contents, which also gives free rein to the scripting of unsubstantiated factoids concerning the book.

If I give my imagination free rein on this sultry afternoon, I could almost believe this shoulder-high grassland – a shimmering sea of false oat, cock’s-foot and meadow foxtail – is African savanna.