Speedkeep

Speedkeep refers to any method that maintains horizontal speed in a situation where it would normally be lost.

With this definition, this would mean Mockball is formally a speedkeep, so perhaps a more restrictive definition is desirable.

The term is frequently conflated with temporary blue suit, which is a method for maintaining the boost counter but potentially losing horizontal speed. Speedkeep methods normally maintain boost counter (e.g., speedballing), but the reverse is not true: Most common temporary blue suit methods lose horizontal speed.

Spike Speedkeep

The most common method for speedkeeping is using an unmorph on a spike. This is done by unmorphing frame perfectly after bouncing, just before taking damage from the spike. This can be done by either landing on a long string of spikes in Morph and taking damage twice (unmorphing before the second hit), or bouncing onto spikes and unmorphing just before hitting them.

Two uses for spike speedkeep in common categories are in Attic and Red Brinstar Fireflea Room.

Moonwalk Speedkeep

After performing a spike speedkeep, you can switch from forwards to backwards on the frame Samus stands up (while holding Shoot) to being moonwalking without losing speed. Given that this requires a frame perfect switch from forwards to backwards after a frame perfect unmorph, and it already having very few uses, this is more a curiosity for RTA applications than actually helpful.

The speed is properly added, though, so this does allow moonwalking at faster speeds which looks silly.

Note by frame perfectly switching from backwards to forwards, the speed is still maintained.

Damage Boost Speedkeep

If a damage boost is initiated as early as possible (in the first two frames), horizontal speed is maintained. From this point, backwards can be released as long as jump is held, and speed will still be maintained.

This technically makes it possible to damage boost, switch to moving forwards, and mockball, all without losing speed.

Other Methods

There may exist other methods, like "grapple speedkeep", but as these all tend to be frame perfect they tend to have very few RTA applications.