Because of the forcing from gravity, spiral arms induce a reverse shear in the stellar rotation, slowing down the stars on the inner parts and speeding up the stars on the outer parts of each arm (Roberts 1969). This reverse often cancels the normal shear from average orbital motions, and makes an arm that has very little internal shear. The arm forcing from gravity also pulls everything in the arm toward the center of the arm, i.e., in a convergent manner, which is opposite to the tidal force from the surrounding galaxy. Thus spiral arms also have reduced galactic tidal forces (Elmegreen 1992). These conditions are good for the formation of large cloud complexes, which are only weakly bound at the start. Giant clouds that form by gravitational instabilities in spiral arms do not immediately shear out into little spirals, and this allows them to grow. When they emerge from the arm, the shear rate and tidal disruption rate increase a lot, and the low-density parts of the clouds can come apart. They can also form feathers and spurs. Such feathering is commonly seen (La Vigne et al. 2006). Feathers occur primarily in grand design (2-arm) galaxies with prominent dust lanes. The feathers are closer where gas density is highest, as expected for gravitational instabilities. Their separation is 5-10 Jeans lengths in the dust lane (La Vigne et al. 2006).
The peculiar motions from spiral arm gravity and Coriolis forces cause the gas and stars to stream along the spiral arms when they are in the arms, and to expand away from the arms when they are between the arms. Streaming motion of the gas can be very strong, perhaps 50 km s-1 or more, as in M51 (Shetty et al. 2007). Radial streaming changes sign at corotation and the observation of this allows one to locate the corotation resonance radius (e.g., Elmegreen et al. 1998). Streaming motions also allow one to measure the timing of the star formation response to the spiral arm (Tamburro et al. 2008). The streaming pattern for gas tends to be inward inside the main parts of the arms inside the corotation radius, and outward in the interarms inside corotation. This pattern of radial motions relative to the arms reverses outside of corotation. Thus the gas and star formation in outer disk spirals, as viewed, for example, by GALEX, should be streaming with positive galacto-centric radial velocities, whereas the gas and star formation in inner disk spirals should be streaming with negative galacto-centric radial velocities. Shetty et al. (2007) found a net radial inward streaming flux for the inner part of M51, including both the arms and the interarms. They suggested that this meant a non-steady spiral pattern.