The video shows the reverse key lock (a.k.a. the kimura, hammerlock or figure four armbar) from two positions, the guard and cross body.

From the closed guard, you can secure the lock and maintain a closed guard but it is often easier to go to open guard and rotate 90º.  This angle (yes, angling works in standup and groundwork) puts your partner in a weaker position to use their strength against you, so long as you secure their torso with a body lock or by simply pushing down with your top leg.

From the cross body position, you’ve already got the advantage of a 90º relative to your partner, so to make matters worse for them and to make it easier to secure the lock, push their ‘other’ arm up by their ear by scooting your hips towards their head.  This will make it very difficult to use that arm in any functional way to fight what you are trying to do.

Once you’ve got the reverse key lock from those two positions, see where else you can set it up from.  Application is all about adaptation, after all.