Danaher says:

Relaxation and tension: One of the toughest things to learn on your Jiu jitsu journey is the juxtaposition of RELAXATION and TENSION as you perform Jiu Jitsu. Relaxation is important for ENDURANCE and EFFICIENT SMOOTH MOVEMENT. If you are overly tense you can’t move well and you’ll fatigue very quickly. On the other hand, isometric tension is important to IMMOBILIZE an opponent in pins and submission holds. If you’re too relaxed while applying pins and submissions an opponent will simply shrug you off and escape. YOU MUST LEARN TO NAVIGATE BETWEEN THE DEMANDS FOR RELAXATION AND TENSION IN ORDER TO BE EFFECTIVE. One without the other will lead you to failure. Look how Gordon Ryan is transitioning rapidly from his usual relaxed state to one of maximal tension as he enters and progresses through a submission attack. He is placing closed wedges via triangle around his opponents head and arm – he will need great muscular tension against ban opponent of this caliber in order to maintain those wedges in place against tremendous physical resistance. When sparring – keep a good sense of when you need to relax and when you need to tighten up. Use tension only in short bursts when either extreme defensive or offensive demand compels you – and the moment it’s over, go back to relaxation.