If the situation weren't so dire, the force with which the Death Eater's wand flew at Sirius might have been amusing. As it was, however, Regulus merely made a mental note not to put the full force of his will behind his spells; here, apparently, there was magic in abundance for them to draw upon.
Aiming at the downed Death Eater, Regulus sent silvery ropes slithering out of the tip of his wand, binding the man securely so that he couldn't re-enter the fray. It was a little thing to do, but a good first step, and Regulus took satisfaction in it.