Momentum is what happens when things or events start happening in the way that are favorable to a company or person. There is a distinct feeling between pre-momentum (things were so damnhard!) and post-momentum (things are so much easier!).
The difference between pre-and post-momentum is simply that external forces are more aligned to what you are doing. Sometimes it's because you shifted the market segment you're targeting, or the market timing is much better now, or you're much better at pitching what you're doing. All those factors and more can be reasons.
When post-momentum happens, it seems important to keep riding and even accelerate. No one knows how long post-momentum will last, so the logical thing to do in an undefined time period is to continue or increase the trajectory. If there is insufficient time, labor, or other resources to capitalize on post-momentum, the logical answer seems to be to hire or find more resources, because that is solvable whereas momentum is not.
Pre-momentum and post-momentum rely on the fundamental same internal work ethic and tempo. That tempo remains fairly constant over time. That tempo is the bedrock, despite what external things happen.
Internal tempo is also important because momentum can disappear. Post-momentum can morph back into pre-momentum, the good times can change to the bad. When that happens, the tempo is the one constant to ride out fluctuating times before the next wave appears.