Godspeed (Shin-soku) is the Superhuman speed of the Hiten Mitsurugi Ryu. A Speed so great it makes the movement of the user appear as a mere flash or blur. A Speed only surpassed by the Mobility technique Shukuchi (Reduced earth) and the Hiten Mitsurugi Ougi 'Amakakaru Ryu no Hirameki'..
Not to be confused with Pseudo-GodSpeed, That Kenshin uses the majority of the series due to the lack of Intent to Slay the enemy or without using the Ougi Amakakaru Ryu no Hirameki (as in the occasions of his fights with Jinei Udo ,Hajime Saito & at the very end of the first fight with Yukishiro Enishi).
Vs Jinei: with Kouru's life on the line, Kenshin's speed increases so dramatically that Jinei Udo actually recognizes the new level of speed & skill as "The Hiten Mitsurugi Style!",
Vs Hajime Saito: kenshin reverts back to his manslayer persona the more he fights Saito. who gets caught completely off guard by kenshin's sudden increase in Speed, twice. Speed surpassing his expectations of what he already knows about Kenshins speed from earlier in their fight.
VS Inishi Yukishiro: after having wounded kenshin so bad he should have been practically immobilized by the Watto jutsu counter of the Amakakeru ryu no herameki, makes kenshin aware he means to kill Kouru for his revenge. Suddenly, inishi is completely overwhelmed by kenshin's new speed & Skill Multiple times, with such ease, that after the third time he literally thinks to himself "this is Ridiculous".
Other then the use of the Hiten Ogi, Amakakaru ryu no Hirameki. Kenshin consistently shows he does not use Godspeed without extreme circumstances & the Intent to seriously harm, if not slay his opponent.
Seijiro Hiko 13th: Other then Kenshin, the only other user of Pseudo-Godspeed would be Seijuro Hiko 13th only while wearing the Hiten ryu Heirloom cloak, due to it's 40KG Manto on the top of the cloak along with built in counter-springs that creates resistance for the arms at the shoulder area (worn by each Seijuro Hiko to keep them strong in peace times as well as keeping their God-like abilities under control).
The fact that Hiko, wearing the 40KG weighted cloak, was able to match Kenshin's speed during their brief training. However, after removing the weighted/resistance cloak. Hiko could use Godspeed as he showed kenshin during his final test of his Godspeed Kuzu ryu-sen against kenshin's Beyond-Godspeed battojutsu/Amakakaru Ryu no hirameki.
The only other times Kenshin uses his true speed (Godspeed), Is when Kenshin uses the Hiten Mitsurugi Ogi, Amakakaru Ryu no Hirameki. But, it is only effective when kenshin is able to bridge the gap of life/death & resolve himself to very seriously injure, if not actually slay his enemy (but without flipping the blade of his reverse blade sword) in order to protect his own life as well as the happiness of those closest to him. Thus allowing Kenshin to use his "true speed, skill, strength", without having to revert to his alternate persona (Bottosai the manslayer) as he did before in order to draw on his true speed, skill, strength and fulfilling his primary reason for seeking out Seijuro Hiko to learn the final techniques of the Hiten Mitsurugi ryu.
Godspeed vs Shukuchi
In the battle between Kenshin and Sojiro Seta. The superior speed of Shukuchi (reduced earth) to the Hiten's Godspeed was displayed by Sojiro. Showing that the Shukuchi at 3 steps short was equal, if not a little greater then Kenshin's Speed. However, Kenshin in such a pacifist state, not having any intent to seriously injure is not capable of using Godspeed or the Hiten sword style as it is supposed to be. Even to the point he gets a clear shot after stiring up anger in Sojiro to the point he expresses it so much before an attack, that kenshins Hiten skill to read emotion and anticipate the enemies next move made sojiro's amazing speed nullified. And kenshin completely pulled the attack, wasting his opportunity to win the battle without the Hiten Ogi. And it was not until Sojiro took kenshin so seriously he was going to use his ultimate attack Shuntensatsu (Instant heaven kill). Forcing kenshin to take him deadly serious & use his ultimate attack. Thus, due to the actual use of Godspeed with the amakakaru ryu no hirameki, by kenshin. It was shown in their final contest of speed that Hiten's Godspeed is only about a step slower then the Shukuchi, so, after it's use with an extra step & battojutsu (Hiten Ogi), Kenshin matched it.
Showing that Godspeed is not anywhere near 3 steps short of the shukuchi as previously demonstrated, but actually just 1.
Godspeed Vs Nerves of Insanity:
During the Battle against Enishi Yukishiro. Enishi showed he had developed extraordinary nerves of size and conductivity that allowed him to surpass kenshin in speed to an extent not seen since he fought Sojiro with the shukuchi. However, the speed attained by the "Nerves of Insanity" are in all likelihood merely God-like and not beyond. only giving Enishi a speed comparable to that of the Hiten Mitsurugi ryu's.