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... ipso praecipiente parumper cessere, ut propiores subsidiis in certamine...

ut...essent: Scipio feigns a retreat, (cessere), so tht (ut) "they (the nearer, propiores) might be closer to reinforcement to be sent in/during the battle. Pripior takes the dat., here the gerundive construction, "subsidiis...summittendis, "reinforcements to be sent."